ROUTE Exam Guide v3.3

1 | P a g e

642-902 Exam Guide No filler.

No hype.

Exam-focused.

“A portable, comprehensive guide with everything you need to get up to speed and pass the ROUTE Exam - the first time.”

www.ccnpguide.com

The Online

CCNP ROUTE

2 | P a g e

Introduction

I started www.ccnpguide.com as a way for me to capture technical notes as I prepared for the three major CCNP Exams – SWITCH, ROUTE, & TSHOOT.

As I began sharing my notes with the world, I immediately started to receive feedback on the SWITCH exam’s focus areas and how difficult it was. What

I realized was that the exam prep resources available (read: Cisco Press Books) were not even covering all of the exam topics, including some that you

were required to configure in live simulation scenarios. First-time fail rates seemed normal and a big part of that was because the some of the simulation

scenarios required you to know some extremely specific protocol configuration details that most network professionals just wouldn’t know off the top of

their heads.

I began to tailor my notes to include topics that were not being covered in “official” exam guides and trimmed down those that just were not necessary.

The feedback was overwhelmingly positive from the online community! The problem is, of course, that the notes were not formatted well for off-line

consumption and didn’t include enough lab/scenario-based examples.

This guide is an answer to the countless requests to create a portable, comprehensive, and exam-focused ROUTE prep guide. I’ve refined the online

notes even more to focus exclusively on exactly what Cisco expects you to know on exam day. I have also included a Simulation Scenario section at the

end.

Here’s my recommendation. Read through this manual a few times and make sure you understand each chapter. After you feel comfortable with the

details in each chapter, go to the Simulation Scenarios section and run through the three scenarios until you can solve them off the top of your head.

That may mean running through them ten times each, but trust me – you’ll thank me when you sit for the test.

If you have questions, exam feedback, or want to reach out to me directly - shoot me an email at [email protected]. I promise you’ll get a

response.

Best of luck.

3 | P a g e

Routing Basics 4

EIGRP 11

OSPF 29

Route Filtering & Redistribution 52

BGP 59

&VPNs IPSec 73

IPv6 77

Simulation Scenarios 88

Shortcuts.

4 | P a g e

Chapter 1:

Routing Basics

Cisco 642 902

5 | P a g e

Static Routes In order for routers to forward packets to remote networks, they must know how to reach them. There are two options: static or dynamic routes.

Static routes are manually configured on each router. They are used for a couple of reasons:

• where there is only a single path to a network (a.k.a. stub network)

• when connecting to an ISP and configuring it as a default (static) route

There are a number of problems with implementing static routes network-wide. Some include:

• failure to scale well

• does not automatically react/recover to changes in the network

• tedious to configure for large networks (see point 1)

The prefix and mask is the destination network and subnet mask. You can use an address to define the IP address of the next hop towards the

destination network or specify a local router interface that the router will use to send traffic out to the destination network. The optional distance

keyword can be used to manually define the administrative distance for the route.

To configure a static route:

R1(conf)# ip route prefix mask address|interface [distance]

6 | P a g e

Static Default Routes One of the most common uses of static routes is for creating a default route. There are often cases when you want to forward packets that are not

defined in a specific route out an interface or towards another router. A common example is when connecting to an ISP. If traffic is destined for an

address range not defined within your organization (i.e. your coworker’s Facebook updates), then it makes sense to configure a default route towards

your ISP or other organization.

Floating Static Routes There are some circumstances when it makes sense to use a static route as a backup to a dynamic routing protocol. In order for this to work, however,

the default administrative distance value on the static route must be raised so it will have a lower priority than the dynamic routing protocol (see

administrative distance section below).

Dynamic Routing Dynamic routing protocols can dynamically respond to changes in the

network. The routing protocol is configured on each router and the

routers learn about both each other and remote networks.

Examples of modern dynamic routing protocols include:

• RIP v1,2 (ok, maybe this isn’t very “modern”)

• EIGRP

• IS-IS

• OSPF

• BGP

To configure a static default route:

R1(conf)# ip route 0.0.0.0 0.0.0.0 address|interface

]

7 | P a g e

Distance Vector vs. Link-State

Distance Vector When routers run a distance vector dynamic routing protocol, they periodically send information about their known routes to their connected neighbors.

This is how the router knows whether changes have been made to the network. They compare their routing table against the information they receive

from their neighbors – if it matches, they’re good. If not, they update their routing tables to reflect the changes.

RIP is an example of a distance vector routing protocol.

Link State Link state routing protocols operate differently. Routers send information about the state of their links to the entire network (or area) that they are a part

of. In this way, each router understands the entire network topology and must run an algorithm every time a network change is announced to recalculate

the best routes throughout the network. This makes link state routing protocols much more processor intensive.

The second major difference in link state routing protocols is that updates are only sent if a change on a router’s link occurs. This helps keep bandwidth

utilization low, unlike distance vector protocols which send out reoccurring updates regardless if a change has occurred.

OSPF and IS-IS are examples of link state routing protocols.

Advanced Distance Vector Advanced distance vector is the title Cisco gives to EIGRP, which borrows the best attributes of both distance vector and link state designs. EIGRP does

not send periodic route information; instead it sends updates only when changes occur (like link state protocols). Also, EIGRP forms neighbor

relationships with its directly connected peers and only updates them – not the entire network (like distance vector protocols).

8 | P a g e

Classful Concepts IP routing protocols are either classful or classless and that determines how they present route information.

Classful Classful routing protocols (like RIPv1) do not include the subnet mask in routing updates. When an update is sent, the packet contains only the major

network information depending on whether it is a class A,B, or C address.

For example, a route to network 172.16.10.0/24 would be

advertised as 172.16.0.0/16 because its classful boundary is a

class B address. Obviously if you have broken your major

network boundaries up into smaller subnets that are more

granular than the major classful boundaries, this will not work

well and that’s the reason almost all modern routing protocols

are classless.

Another classful routing example is illustrated on the right.

Classeless Classless routing protocols (like RIPv2, EIGRP, OSPF, IS-IS, and BGP) include the subnet mask in routing updates allowing for VLSM support and

supernetting.

9 | P a g e

Administrative Distance Routers need a way of determining which path to use to a destination network if two or more routing protocols are in use and both advertise a route.

Administrative distance is Cisco’s answer. Cisco has assigned an administrative distance (AD) to each routing protocol that outlines which protocol a

router will prefer. The AD values can be between 0 and 255 with the lowest values being used for routing.

Default AD values

For example, if router R1 receives a route to network 10.10.200.0 from both EIGRP and OSPF, the router will compare the administrative distance of the

EIGRP-learned route (90), to that of OSPF (110). The router will then add EIGRP’s route to the routing table because its AD is lower (90 < 110).

10 | P a g e

Summary

11 | P a g e

Chapter 2:

EIGRP

Cisco 642 902

12 | P a g e

EIGRP Characteristics Fast Convergence

EIGRP uses the DUAL algorithm to converge very quickly. It does this by

knowing neighbor router’s routing tables and predefining primary and

secondary routes to every destination network.

Triggered Updates

EIGRP uses partial triggered updates to its directly connected neighbors

rather than periodically sharing its entire routing table. This saves link

bandwidth because updates are only sent if a change is incurred, only

the changes are sent in the update, and lastly – the updates are only

sent to a routers’ affected neighbors. Very efficient!

Protocol Independent

Enhanced Interior Gateway Routing Protocol supports more than just

IPv4. It supports IPv4, IPv6, IPX, and AppleTalk.

Multicast

EIGRP sends route updates, hellos, and queries to its neighbors using

the multicast address 224.0.0.10 so end hosts are not affected. Hellos

are sent out every 5 seconds by default to learn about new neighbors

and make sure existing neighbors are still available.

VLSM

Variable length subnet masking is supported by EIGRP because it is a

classless routing protocol. That means subnet masks are included in

route updates.

Terminology

Feasible and advertised distance EIGRP’s DUAL algorithm determines the best route to a particular network by using distance information, known as cost or metric. DUAL determines the

lowest cost path by adding up the cost to the destination network. Neighbors exchange the cost to every route they know of when a neighbor adjacency

is formed. A router then uses that information to calculate their own cost to the same network by adding the cost between themselves and their

neighbor, then adding that to the neighbor’s advertised cost.

13 | P a g e

So, (the cost between neighbors) + (the neighbor’s cost to the destination network) = the total cost to the remote network, or the feasible distance.

The cost the neighbor advertised to the remote network is known as the advertised distance. See the diagram below.

Successor

Think of the successor as the active, or primary, route to a destination for EIGRP. The successor is actually the neighbor router that has the least-cost

path to a destination network (a.k.a. has the lowest feasible distance). Successor routes are added directly to the routing table. You should also know

that multiple successors can exist if they have identical feasible distance values.

Feasible Successor

This is more like the backup route EIGRP chooses to a destination network. The feasible successor feature is what makes EIGRP convergence so unique

and so fast. It always tries to find a backup route. In the event that the successor fails, it can immediately switch over to the feasible successor (backup)

route with very little delay. To qualify as a feasible successor, the AD must be less than the successor’s FD. This helps ensure a loop-free layer 3 path.

14 | P a g e

Tables Neighbor Table

EIGRP discovers neighbors by sending out hellos every 5 seconds. When a routers receives a

hello with the same AS number defined, it forms an adjacency and adds the local interface it

used to reach it as well as the neighbor’s IP address to the EIGRP neighbor table.

Topology Table

When routers form an adjacency, they exchange route information. That information is

transferred to the EIGRP topology table, which contains all the destinations advertised by a

router’s neighbors.

There are two different types of entries in the topology table, active and passive. Now you may

think that the active entry is the preferred or “actively-in-use” route, but surprisingly, the

opposite is true. The route in the topology table that is in the active state signifies that it is

“actively” looking for an alternative path to a destination because the successor has failed and

no FS exists. Obviously this is not an ideal scenario.

If a router’s successor route becomes unavailable, but has a feasible successor – the FS will

immediately become the successor and there is almost no delay incurred. This is the primary

reason EIGRP convergence times tend to be some of the fastest of all the dynamic routing

protocols.

If, however, a router’s successor becomes unavailable and does not have a FS to the

destination, it will send query messages to all of its neighbors asking if they know of a path to

the destination. The neighbors will either respond with a path or forward the query to all of

their neighbor routers until a path is identified and relayed back to the original requester or no more neighbor routers exist. During the time the router is

waiting back for a response, it is unable to forward traffic to the destination network, which can hurt EIGRP’s convergence time.

15 | P a g e

Passive entries represent routes that have at least a single successor and perhaps a feasible successor. They are what you should see in a normal, stable

topology. Notice the “P’s” in the output from the show eigrp topology command below. They indicate that the entries in the EIGRP topology table are in

the passive (read: normal) state.

EIGRP Messages Hello

EIGRP hello packets are sent out every 5 seconds by default using multicast address 224.0.0.10 to maintain and discover neighbor relationships. On

slower (T1 and below) and NBMA links, hellos are sent every 60 seconds to conserve bandwidth.

EIGRP hello packets also contain a hold timer which lets the router know if a neighbor is down. The hold timer is set to 15 seconds normally (~3

unresponsive hellos), and 180 seconds for slower WAN links. When a router receives a hello packet from another router with the same AS (Autonomous

System) number, it automatically forms a neighbor relationship (also known as an adjacency).

R1#sh ip eigrp topology

IP-EIGRP Topology Table for AS(1)/ID(10.1.1.1)

Codes: P – Passive, A – Active, U – Update, Q – Query, R – Reply, r – reply Status, s – sia Status

P 10.1.3.0/24, 1 successors, FD is 156160

via 10.1.100.3 (156160/128256), FastEthernet0/0



via 10.1.200.2 (2297856/128256), Serial1/0


via Connected, Loopback1




via Connected, FastEthernet0/0


via Connected, Serial1/0

16 | P a g e

Update

During the EIGRP start-up process on a router, an update message is

sent out to its neighbors containing the contents of the router’s routing

table. The only other time an update packet is sent is when network

changes occur on a router and it then sends out an update message to

its neighbors who the route change would affect.

Query

When EIGRP loses its successor route and does not have a FS, it sends

out a query message to all of its neighbors asking if they know a path.

(See topology section above)

Ack

Acknowledgement packets are sent in response to update, query, or

reply packets.

Reply

When a router responds to a neighbor router looking for a route (query),

it sends it in the form of a reply.

Graceful Shutdown When an EIGRP process is shut down, the router sends out “goodbye” messages to its neighbors (ironically in the form of hello packets). The neighbors

can then immediately begin recalculating paths to destinations that went through the shutdown router without having to wait for the hold timer to expire.

EIGRP Metrics There are 5 descriptives EIGRP uses to calculate it’s metric, although Cisco generally does not recommend tuning these metrics unless you have a very

specific purpose. You should be aware that only the bandwidth and delay numbers factor into the default formula.

• Bandwidth – the lowest bandwidth value between the source and destination

• Delay – the cumulative delay along a series of links

• Reliability

• Load

• MTU

17 | P a g e

EIGRP Configuration

Define EIGRP as the routing protocol with a predefined Autonomous System ID. Routers will not form a neighbor relationship if their

AS numbers do not match.

Define the attached networks you want to participate in EIGRP

Add each network to the EIGRP process with the network prefix mask command. The mask is an inverted mask, like ACLs use.

Example, a /24 mask would be 0.0.0.255.

The network prefix/mask command tells the router which local interfaces will then participate in EIGRP. This can be very useful if you

do not want specific interfaces to participate in EIGRP.

Using the mask statement will define how you want the routes summarized if you turn off auto summarization. If you choose not to use the mask, EIGRP

will assume the networks are part of the major networks (class A,B,C boundaries) and could cause potential problems.

The output of R3′s running configuration can be seen below.

Step1

Example:

R3(config)# router eigrp 1

Step2

Example:

R3(config-router)#router eigrp 1

R3(config-router)# network 10.1.100.0 0.0.0.225



R3(config-router)# no auto-summary

18 | P a g e

EIGRP Verification

Example:

R3#sh run | begin router eigrp 1

router eigrp 1

network 10.0.0.0

network 192.168.100.0 0.0.0.3

network 192.168.100.4 0.0.0.3

no auto-summary

show ip eigrp neighbors // Displays EIGRP neighbors a router has discovered.

R3#sh ip eigrp neighbors

IP-EIGRP neighbors for process 1

H Address Interface Hold Uptime SRTT RTO Q Seq

(sec) (ms) Cnt Num

1 10.1.100.2 Fa0/0 13 00:12:23 737 4422 0 21

0 10.1.100.1 Fa0/0 14 00:12:29 535 3210 0 22

19 | P a g e

show ip eigrp topology

show ip eigrp topology // Displays the output of the EIGRP topology tables including successor and feasible successor routes.

R3#sh ip eigrp topology

IP-EIGRP Topology Table for AS(1)/ID(192.168.100.5)

Codes: P – Passive, A – Active, U – Update, Q – Query, R – Reply,

r – reply Status, s – sia Status












via Connected, FastEthernet0/0


via 10.1.100.1 (2172416/2169856), FastEthernet0/0 via 10.1.100.2 (2172416/2169856), FastEthernet0/0

show ip route // Shows the IP routing table entries for all routing protocols. R3#sh ip route

Codes: C – connected, S – static, R – RIP, M – mobile, B – BGP

D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area

N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2

E1 – OSPF external type 1, E2 – OSPF external type 2

i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2

ia – IS-IS inter area, * – candidate default, U – per-user static route

o – ODR, P – periodic downloaded static route

20 | P a g e

show ip route

show ip route continued…

Gateway of last resort is not set

10.0.0.0/24 is subnetted, 5 subnets

C 10.1.3.0 is directly connected, Loopback3

D 10.1.2.0 [90/156160] via 10.1.100.2, 00:14:46, FastEthernet0/0


C 10.1.100.0 is directly connected, FastEthernet0/0


[90/2172416] via 10.1.100.1, 00:14:46, FastEthernet0/0




show ip route eigrp // Displays the EIGRP routes that the routing table is using. R3#sh ip route eigrp





[90/2172416] via 10.1.100.1, 00:16:49, FastEthernet0/0

21 | P a g e

Additional EIGRP Configuration Options

EIGRP Default Routes Defaults routes make life easier in many situations. They can decrease the size (and complexity) of the routing table by providing a path to all unspecified

destinations.

One option is to use a static default route with the ip route 0.0.0.0 0.0.0.0 interface/address statement as discussed in the Routing Fundamentals page.

This must be configured on every router that will use that default route.

Another option if you are running EIGRP is to use the ip default-network network-number command in global configuration mode. Any network that

is reachable within the local router’s routing table is eligible to be used by EIGRP as a default route. Once configured, EIGRP will advertise the route to its

EIGRP neighbors as a default route.

** If you want to use this method in conjunction with a static route – you will have to first redistribute the static route into EIGRP.

** Once you use the ip default-network command to define a default route for EIGRP, the router creates a static route in the configuration without

notifying you. That means in order to remove the default route, you must use the no ip route command instead of no ip default-network.

22 | P a g e

Summarization EIGRP summarizes routes by their major classful boundaries, which can be problematic and cause specific subnets to not be advertised correctly.

It is also possible to manually summarize routes with EIGRP out specific interfaces. Under the interface configuration mode, use the ip summary-

address eigrp autonomous-system command:

EIGRP over WAN Networks

EIGRP + MPLS

MPLS defines the customer’s WAN routers as CE, or customer edge routers and the carrier’s border routers as PE, or provider’s edge routers. The CE

routers appear to each other as directly connected peers. When CE West sends information to CE East, PE West intercepts the data, strips the Ethernet

frame, encapsulates it into a MPLS packet, and forwards it over the service provider’s network to PE East. PE East strips off the MPLS information, re-

encapsulates it into an Ethernet frame and forwards it on to CE East.

To disable automatic summarization: R1(config)# router eigrp 1

R1(config-router)# no auto-summary

R1(config)# intferface s0/0/0 R1(config-if)# ip summary-address eigrp 1 10.1.2.0 255.255.255.0

23 | P a g e

This transparent transport allows an EIGRP neighbor relationship to form between the two customer routers.

EIGRP + Frame Relay

Let’s face it; frame relay is a dying WAN technology. Other, more current WAN options like MPLS and metro Ethernet have taken over, but Cisco thinks

it’s important for us to understand the underlying framework of how frame relay works. Frame relay works using switched, virtual circuits through the

service provider network. One of the advantages of Frame Relay is that it allows multiple logical circuits to be configured on a single physical interface.

Each VC is identified with a locally-significant DLCI, or Data-Link Connection Identifier. The layer 2 virtual circuit must then be mapped to a layer three

neighbor, which can be either dynamic or static.

Frame relay is able to emulate point-to-point links by using multiple subinterfaces on a single physical interface (often used on hub-and-spoke topologies).

This allows neighbor’s to be identified as down much more quickly for two reasons:

1. The default timers are shorter (5 sec hold timer, 15 second dead timer).

2. The subinterface is marked down whenever its local DLCI goes down.

24 | P a g e

Static

To configure frame relay statically, configurations must be done on the interface level. The broadcast descriptive is required at the end of the statement

because frame relay defaults to a non-broadcast medium. Also, static mappings can be applied to both multipoint interfaces as well as subinterfaces on a

single physical port.

Dynamic

Dynamic mappings use inverse ARP. In this case, routers only form EIGRP neighbor relationships with other routers they connect to using a frame relay

virtual circuit.

No IP split horizon

When running EIGRP on frame relay multipoint subinterfaces, a major communication problem can occur. Split-horizon is a method of preventing routing

loops in distance-vector routing protocols by prohibiting a router from advertising a route back onto the interface from which it was learned.

When a hub and spoke frame relay topology exists, multipoint subinterfaces are configured on the hub router. The issue is that split horizon is enabled by

default, so in the example below, if R2 learns routes from R1, it cannot then pass those on to R3 because split horizon would prevent the advertisement

from going out the same physical interface. This results in R2 being able to communicate with the spoke router’s networks, but R3 and R1 are unable to

communicate with each other.

To remedy the situation, split horizon must be disabled on the R2’s EIGRP hub interface.

R2(config-if)# no ip split-horizon EIGRP as-number

R1(config-if)# frame-relay map ip remote-ip-address local-dlci broadcast

25 | P a g e

Managing EIGRP Bandwidth

There are two important points to remember when running EIGRP over WAN links. The first is that EIGRP assumes that WAN interfaces run at T1 speed

(1544 kbs). The second is that EIGRP will allocate up to 50% of a link’s bandwidth for EIGRP control traffic.

These two combined can be problematic on links that are slower than a T1 (like a 64k fractional T1 for example). In that situation, EIGRP messages could

choke out data traffic quickly. To control that, the bandwidth command should be used in WAN links to tell EIGRP what the actual link bandwidth is.

EIGRP is often used on frame relay for this reason alone. The ability to control the routing protocol’s usable bandwidth so simply makes it a popular

choice.

More EIGRP options

Passive Interfaces

Not to be confused with the passive (healthy) topology table entries,

interfaces with the passive-interface command applied do not allow any

routing updates or hellos out the interface. For EIGRP, this means that

the router will not form adjacencies with connected routers on that

particular port.

Unicast

EIGRP uses multicast address 224.0.0.10 when sending messages to its

neighbors. You should be aware that EIGRP can also use a unicast

address when communicating with a specific neighbor. To configure it:

The IP address used must be in one of the same subnet ranges as one

of the router’s interfaces.

R1(config)# int serial 0/0/0

R1(config-if)# bandwidth 64

R1(config)# router eigrp 1

R1(config-router)# passive-interface gig 3/1

R1(config)#router eigrp 1

R1(config-router)# neighbor ip-address

26 | P a g e

EIGRP load balancing Out of the box, EIGRP will automatically load balance across equal-cost paths with no special configuration. EIGRP is unique, however, in its ability to

load balance across unequal-cost paths with a single command.

The variance command allows unequal-cost load balancing over up to 6 different paths. But here’s the key, it only works when the cost of the path is

lower than the variance number multiplied by the best metric.

Here is an example scenario.

R1 will by default use the path through R3 because it has the

lowest metric. To enable unequal-cost load balancing, we can

use the following command:

The variance command multiplies the best cost (10,000) by 2

(20,000) and will begin load balancing across all paths with a

FD less than or equal to that – which includes the path

through R2(15,000). This will load balance the traffic in proportion to each path’s metric.

Maximum-paths

By default, Cisco IOS will load balance across 4 equal-cost paths only. Using the

maximum-paths command, you can configure the router to load balance over up to 16

paths. Setting it to 1 disables the load balancing.


R1(config-router)# variance 2

R1(config)# maximum-paths number-of-paths

27 | P a g e

EIGRP Authentication EIGRP supports authentication of its messages using an MD5 hash. When configured, if an incoming EIGRP packet’s hash does not match the local hash,

the packet is silently dropped.

Authentication configuration steps:

1. Configure a key chain to group the keys (read: passwords).

2. Create a key(s) inside the keychain. The router will look inside the keychain and compare the keys against incoming packets.

3. Enable authentication and assign a key to an interface.

4. Indicate MD5 as the authentication type.

Example:

R1(config)# key chain TEST

R1(config-keychain)# key 1

R1(config-keychain-key)# key-string samplepassword

R1(config-keychain-key)# exit

R1(config)# interface gig 1/12

R1(config-if)# ip authentication mode eigrp 10 md5

R1(config-if)# ip authentication key-chain eigrp 10 TEST

28 | P a g e

EIGRP Stub Routing If a router is a spoke in a hub-and-spoke router topology, it is considered a stub router. It is not a transit router and usually has only a single neighbor

router, sometimes two.

Within EIGRP you can define a router as a stub router to limit the EIGRP queries. This saves bandwidth and prevents neighbor routers from requesting

alternate routes when a path fails. If you have many spoke routers, this can dramatically improve EIGRP reconvergence time. The EIGRP stub router still

receives all route updates from its neighbor(s) by default.

EIGRP Stub Options


R1(config-router)# eigrp stub [receive-only | connected | static | summary | redistributed]

Cisco EIGRP Best Practices: • Summarize routes when possible.

• Limit the network depth to 7 hops.

• Limit the scope of EIGRP queries

29 | P a g e

Chapter 3:

OSPF

Cisco 642 902

30 | P a g e

Open Shortest Path First (OSPF)

• OSPF, or Open Shortest Path First, is a link-state, open-standard, dynamic routing protocol. OSPF uses an algorithm known as SPF, or Dijkstra’s

Shortest Path First, to compute internally the best path to any given route.

• OSPF is classless and converges fairly quickly, using cost as it’s metric. A router running OSPF creates its own database which contains information on

the entire OSPF network, not simply neighbor’s routes like EIGRP. This allows the router to make intelligent choices about path selection on its own

instead of relying exclusively on neighbor information.

• OSPF routers do form neighbor relationships though. They exchange hellos with neighboring routers and in the process learn their neighbor’s Router

ID (RID) and cost. Those values are then sent to the adjacency table.

• Every router is responsible for computing its own best paths to all destinations within an OSPF domain. Once the SPF algorithm selects the best

paths, they are then eligible to be added to the routing table.

Link State Database Once a router has exchanged hellos with its neighbors and captured Router IDs and cost information, it begins sending LSAs, or Link State

Advertisements. LSAs contain the RID and cost to the router’s neighbors. LSAs are shared with every other router in the OSPF domain. A router stores

all of its LSA information (including info it receives from incoming LSAs) in the Link State Database (LSDB).

I apologize if the acronyms are starting to pile up. OSPF, architecturally speaking, is more complicated than its counterpart EIGRP – and the long list of

acronyms and definitions is part of that.

31 | P a g e

Areas OSPF is different from EIGRP in that it uses areas to segment routing domains. This helps partition routers into manageable groups if the layer 3 network

begins to get large.

It all starts with area 0. Every OSPF network must contain an area 0, sometimes referred to as the backbone area and every additional area must be

physically connected to area 0. From there, other areas are optional.

Note that the SPF algorithm only runs within a single area, so routers only compute paths within their own area. Inter-area routes are passed using

border routers.

All link state databases must match within an OSPF area. This means

that the more OSPF-enabled routers are configured for the same area,

the more LSA advertisements that must be sent out. After you reach

about 50 routers, the high levels of LSA traffic and numerous routing

table entries can become a problem. That is why Cisco recommends

limiting an OSPF area to no more than 50-100 routers.

The following three factors determine the maximum number

of routers:

• How easily the area’s subnets can be summarized

• The type of areas being used

• The number of external LSAs being injected

An added bonus of partitioning out your OSPF network into areas is

that it is a natural fit for a hierarchical IP scheme.

32 | P a g e

Area Types

Backbone area

Another name for area 0

Regular area

Non-backbone area, with both internal and

external routes

Stub area

Contains only internal routes and a default

route

Totally Stubby Area

Cisco proprietary option for a stub area

Not-So-Stubby area (NSSA)

Contains internal routes, redistributed routes,

and optionally a default route

Totally Stubby NSSA

Cisco proprietary option for NSSA

Router Roles

Internal: All interfaces in a single area (routers 1,4,5 in diagram above)

Backbone: At least one interface assigned to area 0 (routers 1,2,3 in

diagram above)

Autonomous System Boundary Router (ASBR): Has at least one

interface in an OSPF area and at least one interface outside of an OSPF

area.

Area Border Router (ABR): Have interfaces in two or more areas

(routers 2,3 in diagram above)

ABRs contain a separate Link State Database, separating LSA flooding

between areas, optionally summarizing routes, and optionally sourcing

default routes.

33 | P a g e

OSPF Metric Each interface is assigned a cost value based purely on bandwidth. The formula is:

Cost = (100Mbs/bandwidth)

Higher bandwidth means a lower cost!

Let’s run through some common examples quickly:

T1 line | 100,000 / 1544 = 64

10 Mbps | 100,000 / 10,000 = 10

100 Mbps | 100,000 / 100,000 = 1

1000 Mbps | 100,000 / 1,000,000 = .1 1 (OSPF still uses 1 for this, see explanation below)

The cost is then accrued at each hop along the path based on the link’s bandwidth. Unfortunately, OSFP was written when 100Mbs was considered fast.

Because of that, it assigns the same cost to any interface with speeds higher than 100Mbs. To OSPF, a Fast Ethernet interface is weighted the same as a

Gigabit Ethernet interface, both a cost of 1.

To fix that problem, you can use the auto-cost command under the

OSPF process.

Another option is to simply change the cost on a per-interface basis with

the ip ospf cost command (using any number between 1-65,535).

R1(config-router)# auto-cost reference-bandwidth 1000 R1(config-if)# ip ospf cost 35

34 | P a g e

Link State Advertisements LSAs contain a sequence number and a Router ID. Sequence numbers are 32 bits, starting with 0×80000001. The sequence number increases if:

• a route is added or deleted

• a LSA ages out

The largest sequence number is always the most current. The default time that LSAs are aged out is 30 minutes. When an LSA enters a router, it checks

it against its internal Link State Database (LSDB).

• If it is new, it is added to the LSDB and the SPF algorithm is re-run.

• If it contains a Router ID (RID) that is already in the database, entries with an older sequence number are discarded.

• If it receives an older version (according to its sequence number), it discards the LSA and sends back the newer version to the original sender.

The command show ip ospf database will display the sequence numbers and age (in seconds) for each entry.

LSDB Overload In large OSPF networks, if major network changes occur, a flood of LSAs will immediately hit the entire network. The number of incoming LSAs to each

router could be substantial and bring the CPU and memory to its knees.

To mitigate that scenario, Cisco offers what it refers to as Link Sate Database Overload Protection. Once enabled, if the defined threshold is exceeded

over one-minute time period, the router will enter the ignore state – dropping all adjacencies and clearing the OSPF database. Know that this is a drastic

response because routing will be disrupted during that period.

R1(config-router)# max-lsa number

35 | P a g e

LSA Definitions

36 | P a g e

OSPF Messaging OSPF uses several different types of messages to maintain neighbor relationships and correct routing information.

OSPF Packet Types Hello

Discovers neighbors and works as a keepalive.

Link State Request (LSR)

Requests a Link State Update (LSU), see below.

Database Description (DBD)

Contains a summary of the LSDB, including RIDs and sequence numbers.

Link State Update (LSU)

Contains one or more complete LSAs.

Link State Acknowledgement (LSAck)

Acknowledges all other OSPF packets (except hellos).

OSPF sends the five packet types listed above over IP directly, using IP port 89 with an OSPF packet header. Multicast address 224.0.0.5 is used if

sending to all routers, address 224.0.0.6 is used for sending to all OSPF DRs.

OSPF Neighbors Hellos are sent out periodically using multicast on OSPF enabled routers. The router forms an adjacency with a peer router when it sees its own Router

ID in the neighbor field of another router’s hello message. That indicates there is direct, bi-directional communication on the same subnet.

Note: On multi-access links, adjacencies are only formed between the router and the DR and BDR.

37 | P a g e

All of the following fields in an OSPF hello message must match for an adjacency to form:

• hello timer

• dead timer

• area ID

• authentication type

• password

• stub area flag

As with many network protocols, hellos act as a form of keepalive or heartbeat. With OSPF, if four consecutive hellos are not received (the dead time),

the router is considered down.

Point-point interfaces: hellos every 10 seconds, 40 second dead timer

Nonbroadcast multiaccess (NBMA) interfaces: hellos every 30 seconds, 120 second dead timer

38 | P a g e

OSPF States There are 7 different OSPF states when forming neighbor relationships. Take the time to learn the states and their corresponding functions.

Down State

OSPF has not started and no hellos have been sent.

Init State

Hellos are sent out all OSPF-participating interfaces

Two-way State

A hello is received from another router with its own RID in the neighbor field. All other required elements match and the routers become

neighbors.

Exstart State

Routers determine which one will begin the route exchange process with the other.

Exchange State

Routers exchange DBDs.

Loading State

Routers compare the DBD to their LS database. LSRs are sent out for missing or outdated LSAs. Each router then responds to the LSRs with

a Link State Update. Finally, the LSUs are acknowledged.

Full State

The LSDB is completely synchronized with the OSPF neighbor.

1

2

3

4

5

6

7

39 | P a g e

OSPF Configuration OSPF configuration is not too complicated, but has some important syntax distinctions from EIGRP. First, it is configured from router configuration mode

and requires a process ID appended to the router ospf command. The process ID is only locally significant, so don’t worry if it doesn’t match on other

OSPF routers.

The next step is to determine which router interfaces you want participating in OSPF. Just like EIGRP, the network statements define which local router

interfaces will participate.

In the example above, interfaces in the 10.1.1.0/24 subnet will participate in OSPF area 0. Interfaces in the 10.9.9.0/24 subnet will participate in OSPF

area 1. Unlike EIGRP, the subnet wildcard mask in the network statement is not optional because OSPF is classless by default.

Let’s do another example…

R1 has six interfaces, all within area 0:

• GigabitEthernet 0/0: 192.168.100.1/24




• Serial 1/0: 10.100.100.1/30

• Serial 1/1: 10.100.100.5/30

R1(config)# router ospf process-id

R1(config)# router ospf 10

R1(config-router)# network 10.1.1.0 0.0.0.255 area 0


40 | P a g e

The simplest way to configure OSPF an all interfaces into area 0 would be to use this command:

A second option is to break up the 10. and 192. networks into different statements:

The third way to configure the interfaces to participate in OSPF:

All three approaches achieve the exact same result. The configuration you choose is up to you.

Interface Configuration

An alternative configuration option is to configure an interface to

participate in OSPF directly. The [ ip ospf process-id area area-id ]

command takes precedence over the more common network commands.










R1(config)# int gig 0/1

R1(config-if)# ip ospf 10 area 0

41 | P a g e

Router ID The SPF algorithm uses a Router ID to identify hops along a path. The problem, of course, is that routers don’t have a generic “router ID” built in.

The designers of OSPF decided to use the highest IP address assigned to a loopback interface as the Router ID (RID) by default. If no loopback is

configured, it will use the highest IP address assigned to an active interface when the OSPF process begins. OSPF will not change the RID, even if

another interface with a higher IP address comes online unless the OSPF process is restarted. This helps keep the network stable and happy. Note: The

clear ip ospf process command will also force the OSPF process to restart, but will cause an outage – so use it with caution.

Loopbacks are preferred for use as a router ID because they are virtual interfaces and are not affected by links going up and down. To configure a

loopback interface, first create it and assign it an IP address.

Static RIDs

It is also possible to manually define a static Router ID within OSPF with the router-id command.

R1(config)# int loopback 0

R1(config-if)# ip address 10.100.100.1 255.255.255.255


R1(config-router)# router-id 10.100.100.1

42 | P a g e

DRs & BDRs SPF works by mapping all paths to every destination on each router. It uses the RID to identify hops along each path and uses bandwidth as a metric

between those hops. This whole system works really well when routers are connected with point-to-point links and OSPF traffic is simply sent using

multicast address 224.0.0.5.

It doesn’t work well, however, when a router is connecting to multiaccess networks like an Ethernet VLAN. Multiaccess OSPF links require a Designated

Router (DR) be elected to represent the entire segment. Another router is then elected as the Backup Designated Router, or BDR. On that specific

multiaccess segment, routers only form adjacencies with the DR and BDR.

The DR uses type 2, network LSAs to advertise the segment over multicast address 224.0.0.5. The Non-Designated routers then use IP address 224.0.0.6

to communicate directly with the DR.

Elections

1. When the OSPF process on a router starts up, it listens for hellos. If it does not receive any within its dead time, it elects itself the DR.

2. If hellos are received before the dead time expires, the router with the highest OSPF priority is elected as the DR. Next, the same process happens to

elect the BDR.

Note: If a router’s OSPF priority is set to 0, it will not participate in the elections.

3. If two routers happen to have the same OSPF priority, the router with the highest Router ID will become DR. The same is true for BDR.

Once a DR is elected, elections cannot take place again until either the DR or BDR go down. This essentially means that there is no OSPF DR preemption

if another router comes online with a higher OSPF priority. In the case that the DR goes down, the BDR automatically is assigned the DR role and a new

BDR election occurs.

Be aware that a router with a non-zero priority that happens to boots first can become the DR just because it did not receive any hellos when the OSPF

process was started – even though it may have a low OSPF priority.

43 | P a g e

The default OSPF priority is 1 and Cisco recommends manually changing that on routers you want to become the DR and BDR. Remember that DRs are

only used on multiaccess links, so they are only significant on an interface level. A router with two different interfaces connected to two different

multiaccess links will have separate DR elections for each segment.

To set the OPSF priority, use the ip ospf priority command on the interface connected to the multiaccess segment. Values can be between 0-255.

OSPF over the WAN Routing protocols assume both broadcast capabilities and full mesh connectivity on multiaccess networks. For OSPF, there are a few points to consider:

• Full mesh environments can use physical interfaces, but often times subinterfaces are used

• Partial mesh environments should be configured using point-to-point subinterfaces

• Hub-and-spoke environments should elect the hub as the DR or use point-to-point subinterfaces – which don’t require a DR

• Frame Relay and ATM maps should include the broadcast attribute

• In multiaccess environments, the DR and BDR should have full virtual circuit connectivity to all other routers

R1(config)# int gig 0/1

R1(config-if)# ip ospf priority 255

44 | P a g e

Summarization First, it’s important to note that running the SPF algorithm on a router is extremely taxing on CPU resources and can easily consume them all. The reason

is because OSPF has to compute the best path to every destination within its area. Avoiding running the algorithm whenever it isn’t required is a big win.

Summarization has two important benefits for OSPF. It prevents topology changes from being passed outside an area – thus reducing the number of

routers re-running the SPF algorithm. It also consolidates many routes in to a single statement, reducing the memory load and database size on OSPF-

enabled routers.

There are two types of route summarization, inter-area and external.

Inter-area Summarization (LSA Type 3) This occurs on ABRs to summarize routes between areas. This really only works well if the networks contained within an area are subnetted contiguously

so that they can be easily summarized into a single statement.

The new summary route’s cost will be equal to the highest cost route within the summary range. After the command is entered, the router will

automatically create a static route pointing to Null0.

In this example, the summary network 10.100.0.0/16 is summarized from area 2.

External Summarization (LSA Type 5) This occurs on ASBRs for routes that are injected into OSPF via route redistribution. After the command is entered, the router will automatically create a

Example: ABR-R1(config)# router ospf 10 ABR-R1(config-router)# area 2 range 10.100.0.0 255.255.0.0

45 | P a g e

static route pointing to Null0.

In this example, an external network has been summarized into 192.168.0.0/16 and is injected into OSPF via a single type 5 LSA.

OSPF Passive Interfaces Like EIGRP, OSPF supports the use of passive interfaces. The passive-interface interface command disables OSPF hellos from being sent out, thus

disabling the interface from forming adjacencies out that interface.

OSPF Default Routes Default routes are injected into OSPF via type 5 LSAs. There are multiple ways to inject default routes into OSPF, but Cisco recommends using the

default-information originate command under the OSPF routing process.

Example: ASBR-R1(config)# router ospf 10 ASBR-R1(config-router)# summary-address 192.168.0.0 255.255.0.0

R1(config)# router ospf 10 R1(config-router)# default-information originate [always] [metric metric]

46 | P a g e

If the always keyword is not used, OSPF will advertise a default route learned from another source, like a static route. If the always keyword is present, a

default route will be advertised regardless if the route exists in the routing table.

Another option is to use the area range and summary-address commands discussed in the summarization section above. Using these will result in the

router advertising a default route pointing to itself.

Stub and Not-So-Stubby Areas Stub areas are another way to simplify route information that gets advertised. Area 2 in the diagram below shows an example.

The ABR in a stub area drops all external routes and instead

uses a default route of 0.0.0.0 (R3 in this example). That is,

they do not know about any non-OSPF route information outside

their own area.

A Cisco proprietary version of a stub area is a Totally Stubby

Area, or TSA. TSAs do not accept any external routes from

non-OSPF sources AND they do not accept routes from other

areas within their OSPF autonomous system. If a router needs

to send traffic to a route outside of its own area, it sends the

traffic using a default route.

*ABRs use default routes in Stub and Totally Stubby

areas.

47 | P a g e

Stubby areas are made into Totally Stubby Areas by appending the no-summary keyword.

The example above sets area 2 as a totally stubby area. The default-cost command is optional and in this case changed the default route cost from 1 to

8.

Stub Limitations • Virtual links cannot be included

• Cannot include an ASBR

• The stub configuration must be

applied to every router within the

stubby area

• Area 0 cannot be a stub

Bullet point 3 is extremely important! If two routers are connected, but one does not have the stub statement configured, the hello packets will be

dropped and they will not form a neighbor adjacency. Both R3 and R5 above would need the configuration.

NSSAs

Not-So-Stubby Areas, or NSSAs were an addendum to the original OSPF RFC and defined a new special LSA, type 7. NSSAs are very similar to stubby

areas, but they allow the use of ASBRs in the area – something stub areas prohibit.

External routes are advertised by the ASBR as type 7 LSAs and the ABR then converts them into type 5 external LSAs when it advertises them to adjacent

areas.

Example: R5(config)# router ospf 10

R5(config-router)# area 2 stub no-summary R5(config-router)# area 2 stub default-cost 8

48 | P a g e

NSSA is configured using the area area-number nssa command as can been seen in the example below. Using the no-summary keyword turns the

area into a Totally Stubby NSSA. A Totally Stubby NSSA does not accept external or summary routes from other areas.

Lastly, the NSSA ABR does not by default advertise a default route back into the area. The default-information-originate option does just that.

The table below should help recap the different area type behaviors.

R4(config)# router ospf 10 R4(config-router)#area 1 nssa [no-summary] [default-information-originate]

49 | P a g e

OSPF Virtual Links OSPF has strict rules around how areas connect and where they can be located. More specifically, every area must be physically connected to area 0 and

area zero must be ‘contiguous’ – meaning it cannot broken into multiple, connected area 0s.

Virtual links were developed as a band-aid to situations that temporarily must violate those requirements. Virtual links connect areas that do not connect

directly to area 0. It can also connect two area 0s together! Keep in mind that Cisco recommends virtual links be a temporary workaround to a short-

term problem, not a permanent design.

The diagram below illustrates an example when a virtual link could be used. Let’s pretend Company ABC and Company XYZ just announced a merger and

now their corporate networks must do the same. In this case, both routers R1 and R2 have now become ABRs and the virtual link configuration will be

applied to them.

The command area area-number virtual-link router-id is applied to each ABR. Note that the area used in the command is the transit area that the

virtual link resides in. Also, the RID identifies the RID of the OTHER router at the end of the link!

Example: R1(config)# router ospf 20

R1(config-router)# area 1 virtual-link 10.30.30.30

R2(config)# router ospf 20 R2(config-router)# area 1 virtual-link 10.50.50.50

50 | P a g e

OSPF Authentication Out of the box, OSPF does not authenticate its protocol’s messages or route updates. OSPF does, however, support two message authentication options:

• Simple Authentication - using plaintext keys

• MD5 Authentication – using a secure hash

Matching authentication methods and keys must be configured on each interface on a segment. Theoretically, different passwords could be applied to

different router interfaces – the routers on the other ends of those links would just be required to have matching information.

Simple Authentication Example:

R1(config)# int fa0/1

R1(config-if)# ip ospf authentication-key KEY123

R1(config-if)# ip ospf authentication

R1(config-if)# exit


R1(config-router)# area 0 authentication

MD5 Authentication Example:

R1(config)# int fa0/1

R1(config-if)# ip ospf message-digest-key 1 md5 KEY123

R1(config-if)# ip ospf authentication message-digest

R1(config-if)# exit


R1(config-router)# area 0 authentication message-digest

** The 1 in the ip ospf message-digest-key 1 md5 KEY123 statement above is a key number.

51 | P a g e

OSPF Verification The OSPF neighbor table can be viewed using the show ip ospf neighbor command. It shows the status of the OSPF database loading process, status of

neighbor adjacencies, as well as DR and BDR assignments.

• To show which OSPF routers are being used by the routing table, issue the show ip route ospf command.

• The show ip ospf command displays the RID, counters, and timers.

• To see which router interfaces are participating in OSPF (and their area assignments), use the show ip ospf interface command.

52 | P a g e

•

Chapter 4: Route Filtering & Redistribution

Cisco 642 902

53 | P a g e

Redistribution is necessary when routing protocols connect and must pass routes between the two. This can happen in a number of situations, but some

examples include:

• Organizations transitioning routing protocols

• Businesses merge, and so must their networks

• OSPF or EIGRP is used at the access and distribution layer of an enterprise and BGP is used in the core

The challenge to redistributing routing protocols is that each routing protocol uses its own metric and they are not compatible with each other.

Furthermore, there is no magic algorithm than can automatically translate metrics between, say RP and BGP.

To deal with this dilemma, a new seed metric is used as a starting point when redistribution is configured.

Configuring Redistribution To configure redistribution between routing protocols, the redistribute protocol command is used under the routing protocol that receives the routes.

The process-id field above is the AS number for RIP, EIGRP, BGP. For OSPF, use the process ID. Also, both RIP and EIGRP require the use of the metric

keyword!

R1(config-router)# redistribute protocol [AS or process-id] [metric metric-value]

54 | P a g e

The example above shows OSPF being redistributed into EIGRP with a metric of 1000 100 255 1 1500. That is a lot of different numbers for an EIGRP

cost! That’s because EIGRP redistribution metric requires you to input all of the metric calculation manually:

• bandwidth

• delay

• reliability

• loading

• MTU

You can perform a show interface on the outgoing router interface prior to see what values the router is currently using.

The example above redistributes EIGRP routes into OSPF. The subnets keyword at the end of the redistribute command is extremely important! Without

this keyword, OSPF will redistribute networks at their classful boundaries – not something most administrators want.

If you don’t use it the IOS will even give you a warning. Make sure to include it.

EIGRP Redistribution Example:

R1(config)# router eigrp 10 R1(config-router)# redistribute ospf 20 metric 1000 100 255 1 1500

OSPF Redistribution Example:

R1(config)# router ospf 100 R1(config-router)# redistribute eigrp 10 subnets

55 | P a g e

Distribute Lists Distribute lists are access lists applied to the routing process, determining which networks are allowed into the routing table or included in updates. They

essentially act as a filter.

Think: access list applied to routing = distribute lists

When creating a distribute list, use the following steps:

Identify the network

addresses to be filtered

and create an ACL –

permitting the networks

you want to be

advertised.

Determine if you want

to filter updates

coming into the router

or leaving the router.

Assign the ACL using

the distribute-list

command.

Incoming Distribute List R1(config-router)# distribute-list {acl-number | name} in [interface-type number]

Outgoing Distribute List R1(config-router)# distribute-list {acl-number | name} out [interface-name | routing-process | AS-number]

Step1

Step2

Step3

56 | P a g e

Route Maps When a routing update arrives at an interface, a series of steps occur to process it correctly. The

diagram below outlines those steps and serves as a foundation for the rest of this route

redistribution and filtering section.

Route maps are extremely flexible and are used in a number routing scenarios including:

• Controlling redistribution based on permit/deny statements

• Defining policies in policy-based routing (PBR)

• Add more mature decision making to NAT decisions than simply using static translation

• When implementing BGP PBR

Route maps allow an administrator to define specific traffic and then take automated actions

against it to control how routing information is processed and forwarded. Route maps uses logic

similar to if/then statements in simple scripting.

In route map terms, it matches traffic against conditions and sets options for that traffic.

NOTE: If you have downloaded the Switch Exam Guide, you will notice the similarity between the

syntax structure of route maps and VACLs.

Each statement in a route map has a sequence number, which are read from lowest to highest.

The router stops reading statements when it reaches its first matching statement.

Understand that there is an implicit deny included in all route maps. If traffic does not match any

statement, it is denied.

57 | P a g e

Basic Route Map Configuration

That is how all route maps begin. Permit means that any traffic matching the match statement that follows is processed by the route map. Deny means

that any traffic matching the match statement that follows is NOT processed by the route map. Remember the difference.

Match & Set Conditions • If no match condition exists, the statement matches anything (similar to a ‘permit any’).

• If no set condition exists, the statement is simply permitted or denied with no additional changes made.

• If multiple match conditions are used on the same line, it is interpreted as a logical OR. In other words, if one condition is true, a match is made.

For example, the router would interpret ‘match a b c’ as ‘a or b or c’.

If multiple match conditions are used on consecutive lines, it is interpreted as a logical AND. In other words, all conditions must be true before a match is

made. A router would interpret the following example as match a and b and c:

R1(config)# route-map {tag} permit | deny [sequence_number]

route-map EXAMPLE permit 5

match a

match b

match c

58 | P a g e

Important route redistribution match conditions

ip address

Refers to an access list that permits or denies networks

ip address prefix-list

Refers to a prefix list that permits or denies prefixes

ip next-hop

Refers to an access list that permits or denies IP next hops IP addresses

ip route-source

Refers to an access list that permits or denies advertising router IP

addresses

length

Permits or denies packets based on length (in bytes)

metric

Permits or denies routes with specific metrics from being redistributed

route-type

Permits or denies redistribution based on the route type listed

tag

Routes can be labeled with a number that identifies it

Important route redistribution set conditions

Metric

Sets the metric for redistributed routes

Tag

Tags a route with a numbered identifier

Route Map Verification Use the show route-map command to verify route maps and PBR entries are filtering as expected.

59 | P a g e

Chapter 5:

BGP

Cisco 642 902

60 | P a g e

BGP, or Border Gateway Protocol is an external, dynamic routing protocol. It is most often used between ISPs and between enterprises and their service

providers. BGP is literally the routing protocol of the Internet because it connects independent network together, enabling end-to-end transport.

Scalability and stability are BGP’s focus, not speed – as a result it behaves very differently than most other routing protocols.

BGP is recommended whenever multihoming is a requirement (dual ISP connections to different carriers), when route path manipulation is needed, and in

transit Autonomous Systems.

A Quick Overview • Routers running BGP are called BGP speakers.

• BGP uses autonomous system numbers to keep track of different administrative domains. 1-64511 are public, 64512-65535 are private.

• BGP is used to connect IGPs, interior gateway protocols like OSPF and EIGRP. Routing between Autonomous Systems is referred to as

interdomain routing.

• The administrative distance for eBGP routes is 20, iBGP is 200.

• BGP neighbors are called “peers” and must be statically assigned.

• Peers receive incremental, triggered updates as well as keepalives using TCP port 179.

• BGP is sometimes referred to as a “path-vector” protocol because its route to a network uses AS numbers on the path to the destination.

• BGP uses its path-vector attributes to help in loop prevention. When an update leaves an AS, the AS number is prepended to update along with

all the other AS numbers that have spread the update. When a BGP router receives an update, it first scans through the list of AS numbers. If it

sees its own AS number, the update is discarded.

61 | P a g e

BGP Databases Like most modern routing protocols, BGP has two separate databases – a neighbor database and a BGP-specific database.

Neighbor Database

Lists all of the configured BGP neighbors (to view – #show ip bgp summary).

BGP Database

Lists all networks known by BGP along with their attributes. (to view – #show ip bgp).

BGP Message Types There are four different BGP message types.

Open

After a BGP neighbor is configured, the router sends an open message to

establish peering with the neighbor.

Update

The type of message used to transfer routing information between

peers.

Keepalive

BGP peers sends keepalive messages every 60 seconds by default to

maintain active neighbor status.

Notification

If a problem occurs and a BGP peer connection must be dropped, a

notification message is sent and the session is closed.

Internal vs. External iBGP, or internal BGP is a peering relationship between BGP routers within the same autonomous system. eBGP, or external BGP describes a peering

relationship between BGP routers in different autonomous systems. It is an important distinction to make.

In the diagram below, R1 and R2 are eBGP peers. R2 and R3 and iBGP peers.

62 | P a g e

BGP Next-Hop Self When you have BGP neighbors peering between autonomous systems

like R1 and R2 above, BGP uses the the IP address of the router the

update was received from as its “next hop”. Routers that receive an

update from an eBGP neighbor must pass the update to its iBGP

neighbors with-out modifying the next hop attribute.

The next-hop IP address is the IP address of the edge router

belonging to the next-hop autonomous system.

For example, let’s say R1 sends an update to R2 from its 10.1.1.1

serial interface. R2 must keep the next-hop IP set as 10.1.1.1 when it

passes the update along to R3, its iBGP peer. The problem is that R3

does not know about 10.1.1.1 and so it cannot use it as its next hop

address.

The neighbor [IP address] next-hop-self command solves the problem by advertising itself as the next-hop address. In this example, it would be

applied to R2 so any updates passed along to R3 would use an R2 address as the next-hop.

R2(config)# router bgp 65300

R2(config-router)# neighbor 10.2.2.2 next-hop-self R2(config)# exit

63 | P a g e

BGPs Synchronization Rule The BGP synchronization rule states that a BGP router cannot use or forward new route updates it learns from iBGP peers unless it knows about the

network from another source, like an IGP or static route.

The idea is to prevent using or forwarding on information that is unreliable and cannot be verified. Remember, BGP prefers reliability and stability over

using the newest, fastest route.

This means that iBGP peers will not update each other unless an IGP is running under the hood. To remove the limitation, use the no synchronization

command under BGP configuration mode. Recent versions of IOS have it disabled by default, but it is important topic to understand.

Resetting BGP Sessions Internet routers running BGP have enormous routing tables. When a filter is applied, like a route map, changes to BGP attributes occur. Those changes

could affect many of the routes already in the routing table from BGP. Because BGP’s network list is usually very long, applying a route map or prefix list

after BGP has converged can be disastrous. The router would have to check the filter against every possible route and attribute combination.

To make matters worse, if it were to apply the filters and pull routes back from neighbors, those changes could then cause another reconvergence – and

on and on. In an effort to avoid that scenario (BGP loves stability), BGP will only apply attribute and network changes to routes AFTER the filter has been

applied. All existing routes stay unchanged.

If the network administrator decides that the filter needs to be applied to all routes, then the BGP instance must be reset – forcing the entire BGP table to

pass through the filter. There are three ways to do this:

• Hard reset

• Soft reset

• Route refresh

64 | P a g e

The hard and soft reset options aren’t discussed here because they are not directly relevant to the exam. You should know though, that both options are

extremely memory-taxing on the router as all the routes must be recomputed. Route refresh was developed to solve the high memory problems, while

still forcing a reset.

The clear ip bgp [ * | neighbor-address] in command performs the BGP route refresh.

BGP Configuration

Enabling BGP Like other routing protocols, BGP must be enabled with the router command. Make sure to include the AS number.

BGP Peering Each neighbor must be statically assigned using the neighbor command. If the AS number matches the local router’s, it is an iBGP connection. If the AS

number is different, it is an eBGP connection.

If a router has a long list of directly connected neighbors, the BGP configuration can start to get long and difficult to follow – especially as neighbor

policies are applied. Peer groups solve that.

R1(config)# router bgp autonomous-system-number

R1(config-router)# neighbor ip-address remote-as autonomous-system-number

65 | P a g e

Peer groups are groups of peer neighbors that share a common update policy. Updating an entire group of neighbor statements can then be done with

one command. Much easier for large BGP networks. Think of a peer group as a logical grouping of routers that are grouped under a single name to

make changes faster and configurations shorter. Like OUs in Active Directory.

Peer groups not only reduce the number of lines of configuration, but they reduce overhead on the router. A BGP update process normally runs for each

neighbor. If a peer group is configured, a single update process runs for all routers in the group. Notice that this means that all of the router inside a

peer group must be either all iBGP or eBGP neighbors.

Basic Neighbor Configuration Example:


R1(config-router)# neighbor 10.1.1.1 remote-as 65300

R1(config-router)# neighbor 10.1.2.1 remote-as 65300 R1(config-router)# neighbor 10.1.3.1 remote-as 65300

Peer Group Configuration Example:


R1(config-router)# neighbor MINE peer-group

R1(config-router)# neighbor MINE remote-as 65300

R1(config-router)# neighbor 10.1.1.1 peer-group MINE



66 | P a g e

BGP Source Address R1 in the diagram has two different options when it comes to

peering to R2. It can peer to the physical interface IP address,

10.1.1.2 or it can peer to R2′s loopback interface, 192.168.2.2.

If a peer relationship is made using the physical interface as the

source address, problems can occur if the interface goes down.

In this scenario, even if R2′s 10.1.1.2 interface drops, it still has

connectivity to R2′s networks via R3 and R2′s other physical

interface. Even though an IGP would still show R2′s network as

accessible, the BGP peer relationship would drop because R1

cannot reach its peering address with R2.

Most implementations recommend using a loopback address as

the BGP source address for this reason. Remember that the

loopback address must be added to the IGP running for this to

work. This way, if R2′s 10.1.1.2 interface fails, R2 will still be

reachable.

The update-source command accomplishes this. Here’s

an example:



R1(config-router)# neighbor 192.168.2.2 update-source loopback0


R2(config-router)# neighbor 192.168.1.1 remote-as 65400 R2(config-router)# neighbor 192.168.1.1 update-source loopback0

67 | P a g e

Defining Networks Network statements in BGP are used differently than in other routing

protocols like EIGRP or OSPF. EIGRP and OSPF use the network

statements to define which interfaces you want to participate in the

routing protocol process.

BGP uses network statements to define which networks the local router

should advertise. Each network doesn’t have to be originating from the

local router, but the network must exist in the routing table. The

optional mask keyword is often recommended as BGP supports

subnetting and supernetting.

Understand that by default a BGP router will not advertise a network

learned from one iBGP peer to another. This is why iBGP is not a good

replacement for an IGP like EIGRP and OSPF.

BGP Path Selection Unlike most other routing protocols, BGP is not concerned with using the fastest path to a given destination. Instead, BGP assigns a long list of attributes

to each path. Each of these attributes can be administratively tuned for extremely granular control of route selections.

BGP also does not load balance across links by default. To select the best route, BGP uses the criteria in the following order:

1. Highest weight

2. Highest local preference

3. Routes originated locally

4. Path with the shortest AS path

5. Lowest origin code ( i < e < ? )

6. Lowest MED

7. eBGP route over iBGP route

8. Route with nearest IGP neighbor (lowest IGP metric)

9. Oldest route

10. Neighbor with the lowest router ID

11. Neighbor with the lowest IP address

Example:




R1(config-router)# neighbor 10.1.2.1 remote-as 65300 R1(config-router)# network 10.1.2.0 255.255.255.0

68 | P a g e

Controlling Path Selection The most common method of controlling the attributes listed above is to use route maps. This allows specific attributes to be changed on specific routes.

Before we get into route maps, let’s first discuss the three prominent attributes, weight, local preference, and MED.

Weight

On Cisco routers, weight is the most influential BGP attribute. The weight attribute is proprietary to Cisco and is normally used to select an exit interface

when multiple paths lead to the same destination. Weight is local and is not sent to other routers. It can be a value between 0-65,535. 0 is the default.

In the example below, if you want R2 to prefer to use R1 when sending traffic to 192.168.20.0 then the weight attribute could be raised on R2 for R1.



R2(config-router)# neighbor 10.2.2.1 remote-as 65100 R2(config-router)# neighbor 10.1.1.1 weight 100

69 | P a g e

Local Preference

• Local preference is not proprietary to Cisco and can be used in a similar fashion to weight. It can be set for the entire router or for a specific

prefix.

• Local preferences can range from 0-4,294,967,295, with 100 being the default value. Unlike weight, local preference is propagated to iBGP

neighbors.

• Using the diagram above, if an administrator wanted R2 to use R1 when sending traffic to 192.168.20.0, the configuration would look something

like this:

After the local preference is raised on R1, it will be shared with R2 and R2 will begin using it as its best path to the distant network (assuming the weight

is the same of course).

If you want to set the local preference on specific prefixes, route maps are usually the best option. Below is an example of the local preference being set

using a route map:

R1(config)# router bgp 65100 R1(config-router)# bgp default local-preference 500



R7(config-router)# neighbor 10.10.10.1 route-map lp_example in

R2(config-router)# exit

R7(config)# access-list 7 permit 10.30.30.0 0.0.0.255

R7(config)# route-map lp_example permit 10

R7(config-rmap)# match ip address 7

R7(config-rmap)# set local-preference 300

R7(config-rmap)# exit

R7(config)# route-map lp_example permit 20

R7(config-rmap)# set local-preference 100

70 | P a g e

MED The MED attribute, or multi-exit discriminator, is used to influence which path external neighbors use to enter an AS. MED is also much farther down on

the attribute list compared to attributes like weight, local preference, AS path length, and origin. The default MED value is 0 and a lower value is

preferred.

A common scenario for MED is when a company has two connections to the same ISP for internet. Weight or local preference could be used to send

outgoing traffic on the higher bandwidth link, but local preference is not shared with routers outside an AS. MED could be set on one router so ISP

routers prefer that path in.

To set the MED on all routes:

Here’s an example using a route map to influence incoming paths to 10.30.30.0/24 using MED:

R1(config-router)# default-metric value



R7(config-router)# neighbor 10.10.10.1 route-map med_example out


R7(config)# access-list 7 permit 10.30.30.0 0.0.0.255

R7(config)# route-map med_example permit 10

R7(config-rmap)# match ip address 7

R7(config-rmap)# set metric 50

R7(config-rmap)# exit

R7(config)# route-map med_example permit 20

R7(config-rmap)# set metric 150

71 | P a g e

Verification It’s important that you understand and are able to interpret the show ip bgp command output. It displays the contents of the local BGP topology

database- including the attributes assigned to each network. It is perhaps the most important BGP verification and troubleshooting tool!

Because BGP uses many attributes and sources routes in a number of ways, the output of the show ip bgp command can be a bit overwhelming if you

don’t know what you are looking for.

Attributes Here’s a breakdown of some important fields you should consider remembering:

* An asterisk in the first column means that the route has a valid next hop.

s (suppressed) BGP is not advertising the network, usually because it is part of a summarized route.

> Indicates the best route for a particular destination. These will end up in the routing table.

R1# show ip bgp

BGP table version is 21, local router ID is 10.0.22.24

Status codes: s suppressed, d damped, h history, * valid, > best, i – internal

Origin codes: i – IGP, e – EGP, ? – incomplete

Network Next Hop Metric LocPrf Weight Path

*> 10.1.0.0 0.0.0.0 0 32768 ?

* 10.2.0.0 10.0.22.25 10 0 25 ?

*> 0.0.0.0 0 32768 ?

* 10.0.0.0 10.0.22.25 10 0 25 ?

*> 0.0.0.0 0 32768 ? *> 192.168.0.0/16 10.0.22.25 10 0 25 ?

72 | P a g e

i (internal) If the third column has an i in it, it means the network was learned from an iBGP neighbor. If it is blank, it means the network was learned

from an external source.

0.0.0.0 The fifth column shows the next hop address for each route. A 0.0.0.0 indicates the local router originated the route (examples include a

network command entered locally or a network an IGP redistributed into BGP on the router)

Metric (MED value) The column titled Metric represents the configured MED values. Recall that 0 is the default and if another value exists, lower is

preferred.

i/e/? The last column displays information on how BGP originally learned the route and are referred to as origin codes.

i - Entry originated from an Interior Gateway Protocol (IGP) and was advertised with a network router configuration command.

e - Entry originated from an Exterior Gateway Protocol (EGP).

? - Origin of the path is not clear. Usually, this is a router that is redistributed into BGP from an IGP.

73 | P a g e

Chapter 6:

VPNs & IPSec

Cisco 642 902

74 | P a g e

R1(conf)# ip route prefix mask address|interface distance_value

VPN tunnels and IPSec are two topics covered on the exam, but not in great detail. You’ll need to know enough to verify a sample configuration and

answer straightforward questions on both technologies. Let’s start with IPSec.

IPSec Basics IPSec allows the establishment of a secure connection between two hosts. The IPSec protocol sets up a unidirectional SA (security association between

the two endpoints). Because the association is unidirectional, an SA is created on both ends, resulting in two SAs per IPSec tunnel.

IPSec tunnels are often used as a backup to a WAN link failure. If a point-to-point WAN circuit drops, an IPSec tunnel can be configured to automatically

be established over the internet to the remote site. When the primary WAN circuit comes back up, the IPSec tunnel is disconnected.

Floating Static Routes

Configuring an IPSec tunnel to activate when a primary link drops is commonly implemented as a floating static route. The idea is to configure to IPSec

VPN as a static route, but with an administrative distance higher than that of the WAN routing protocols’s AD. If the primary route is active, the backup

link is not placed into the routing table because its AD is higher. If the primary route goes down, the static route becomes active.

To configure a floating static route, make sure you define a higher AD value at the end:

VPN Tunnels

One major problem with standard IPSec sessions is that they do not support broadcast or multicast traffic. If you want to use an IPSec VPN in an “always

on” fashion, then the tunnel needs to allow routing information to pass through. Of course dynamic routing protocols use broadcast or multicast to send

hellos and updates, creating a dilemma.

To get around this issue, a “tunnel within a tunnel” approach can be used. A generic tunnel can be configured within the IPSec tunnel to allow routing

protocol information (along with all the other traffic). There are generally four ways to do this paired with IPSec:

75 | P a g e

DMVPN and GET VPN

Both allow the creation of secure, “on-demand”, multipoint tunnels.

Virtual Tunnel Interface (VTI)

A secure, “always-on” tunnel that supports multicast traffic. This allows

routing protocols to operate within it.

Generic Routing Encapsulation (GRE)

GRE tunnels may be the most common of the bunch – they are also the

default tunnel mode on Cisco routers. GRE tunnels support many layer 3

protocols but perhaps most importantly allow multicast traffic across the

tunnel – granting dynamic routing protocol traffic.

Be aware that GRE tunnels add an additional 20 byte IP header as well as a 4 byte GRE tunnel header.

Branch Office Connectivity The CCNP ROUTE exam covers several unusual topics related to managing and configuring the connectivity between an HQ site and a branch office. You

need to be familiar with some of the underlying technologies used.

Cisco ISR routers are often a good choice for branch sites as they support a wide variety of incoming services. In smaller offices, a single ISR may be

used for a both remote connectivity and inter-VLAN routing. In that case, know that an Ethernet Switch Module would be required for the ISR router.

DSL

DSL, or Digital Subscriber Line, can be used as a backup WAN connection to a branch office. DSL uses frequencies not used by TDM phone systems on a

phone line – allowing the extra bandwidth to be used for data connectivity. Asymmetrical DSL has higher downstream bandwidth than upstream, while

with symmetric DSL they are both the same rate. There are two primary methods for pushing L2 data across a DSL line:

PPPoE

Point-to-Point Protocol over Ethernet is the most common method and

encapsulates PPP traffic into Ethernet frames.

PPoA

Point-to-Point Protocol over ATM is less common and routes PPP traffic

over an ATM network between the customer and the DSL service

provider.

** Both options can be configured on a Cisco router to terminate the DSL connectivity. PPPoE is especially helpful because it this frees the user

computers from running PPPoE

76 | P a g e

Cable

Broadband cable providers also provide internet connectivity which can be used for WAN backup or Internet connectivity for telecommuters. The internet

signal is carried on the same line that the television is carried, but a cable modem allows the data traffic to be separated.

The international standard for sending data over a cable system is Data Over Cable Service Interface Specification (or DOCSIS). Many different versions

of the standard are used throughout the world.

Cable system connections are typically not terminated directly into a Cisco router. Instead, a cable modem demodulates the incoming signal and converts

the traffic to Ethernet frames, which a router can process.

77 | P a g e

Chapter 7:

IPv6

Cisco 642 902

78 | P a g e

IPv6 IPv6 is an important topic – and not just for the exam. The growth of web-based services and diminishing IPv4 addressing will continue to push

organizations towards IPv6, especially on web-facing networks.

Basics IPv4 addresses are 32 bits long and are represented in dotted-decimal format. IPv6 addresses are 128 bits and are in hexadecimal format.

The first 64 bits of an IPv6 address are reserved for the network portion and the last 64 bits are used for the host portion.

79 | P a g e

IPv6 Shorthand The ability to shorten IPv6 addresses is very important to understand because it makes reading and writing them much easier. There are two ways to

condense an IPv6 address.

1. Leading zeros can be removed in any section.

0021:0001:0000:030A:0000:0000:0000:0987E

Can be abbreviated as:

21:1:0:30A:0:0:0:987E

2. Sequential sections of all zeros can be shortened to a single

double colon.

This can only be used once per address though! Using the same

example address above, it can be further shortened to:

21:1:0:30A::987E

Unicast, Multicast, & Anycast Unicast

Unicast is for sending traffic to a single interface. In IPv6 there are actually two different unicasts types, global unicast and link-local unicast.

Multicast

Unlike IPv4, IPv6 addressing does not support broadcasts. Instead, it has replaced it with multicast (which is a more efficient variation). This is used for

sending traffic to a group of devices.

Anycast

IPv6 supports another, new packet type – anycast. Anycast allows the same address to be used on multiple devices for load balancing and redundancy.

Technically, it is used for sending traffic to the nearest interface in a group. While multiple devices may be running the same anycast address, only one

will be used per packet sent.

80 | P a g e

Be aware that with IPv6, an interface can be assigned multiple addresses. Here is the list:

• Unicast address

• Link-local address

• loopback (::1/128)

• All nodes multicast (FF00::1)

• Site-local multicast (FF02::2)

• Solicited-nodes multicast

• Default Route (::/0)

Address Assignment In IPv6, there are three different ways devices are assigned an IP address:

• manual configuration

• stateless autoconfiguration

• DHCPv6.

Manual Address Configuration The first thing to know about manual IPv6 address configuration is that addresses assigned to a router interface use the address/prefix-length notation

instead of the address mask notation. This is so much easier than typing 255.255… after every IP address!

• Make sure you first enable IPv6 routing with the ipv6 unicast-routing global configuration command.

• Use the ipv6 address ipv6-address/prefix-length command to assign an address.

An example of an interface configured with an IPv6 address:

R1# conf t

R1(config)# ipv6 unicast-routing

R1(config)# int gig 1/1 R1(config-if)# ipv6 address 21:1:0:30A::987E/64

81 | P a g e

Manual Network Assignment

Another way to manually configure an IPv6 address is to configure the network and allow the host portion to be auto-populated based on the device’s

MAC address. This can work well because MAC addresses are 64 bits long – the exact same length as the host portion of an IPv6 address!

An example with the network portion defined:

Note: Some systems have a 48 bit MAC address. In this case, it flips the 7th bit and inserts 0xFFEE into the middle of the MAC address. This modified

version is called an EUI-64 address. To do this, add the keyword eui-64 to the end of the ipv6 address statement.

Stateless Autoconfiguration Stateless autoconfiguration allows a device to self-assign an IP address for use locally without any outside information. Remember that interfaces using

IPv6 will often have more than one IPv6 address assigned, and in this case stateless autoconfiguraiton will generate a link-local address in addition to any

other manually assigned addresses. Link-local addresses are created by starting with the prefix FE80:: and appending the device’s MAC address. Since

every MAC address should be unique, it works well for auto-generated local IP addresses.

Link-local addresses are not routable within packets and are used for administrative purposes within the local segment. For example, most IGPs use link-

local addresses to for neighbor relationships and the link-local address is listed as the next-hop address in the routing table.

Once a router has created an IPv6 link-local address using stateless autoconfiguration, it uses NDP to make sure it is actually unique within the local

network. NDP stands for Neighbor Discovery Protocol. NDP uses ICMP packets as part of the neighbor discovery process.

To configure stateless autoconfiguration, use the ipv6 address autoconfig command.

R1(config)# int gig 1/1 R1(config-if)# ipv6 address 21:1:0:30A::/64

R1(config)# int gig 1/1 R1(config-if)# ipv6 address autoconfig

82 | P a g e

IPv6 Routing

Static Routes The configuration for IPv6 static routes is identical to IPv4, except for the ipv6 route keywords (instead of ip route) and the addresses will obviously look

different. Other than that, it is exactly the same!

An example of a static IPv6 default route:

An example of an IPv6 static route with a next-hop address:

To view the IPv6 routes in the routing table, use the command show ipv6 route.

IPv6 EIGRP • There are many differences in the way EIGRP is configured and with IPv6. It still sends

hellos out every 5 seconds to its neighbors, but when running EIGRP with IPv6 addresses, it

uses the multicast address FF02::A.

• EIGRP messages are exchanged using the link-local address as the source address and

perhaps the biggest difference is that there is no network command! Instead, EIGRP

routing is enabled on each participating interface.

• Also, EIGRP running IPv6 requires a router ID be configured. The format is that of an IPv4

address - 32 digits and it can be a private address (non-routable) with no issues.

• The last major change is that the EIGRP process starts in the shutdown state. You have to issue a no shut to bring it up on the router.

R1(config)# ipv6 route ::/0 serial1/1 R1(config)# ipv6 route 2003:2:1:A::/64 2003:2:1:F::1

To Configure IPv6 EIGRP:


R1(config)# ipv6 router eigrp AS

R1(config-rtr)# router-id ipv4-address

R1(config-rtr)# no shut

R1(config-rtr)# exit

R1(config)# interface type number R1(config-if)# ipv6 eigrp AS

83 | P a g e

OSPFv3 • OSPFv3 is an updated version of OSPF designed to accommodate

IPv6 natively. Most of the configuration and function is identical to

its predecessor, but a few changes were made – starting with

messaging.

• OSPFv3 uses the multicast address FF02::5 and FF02::6, but like

EIGRP, it now uses its link-local address as the source address in

advertisements. Also, it’s possible to run multiple instances of

OSPFv3 on each link.

• Like the IPv6 implementation of EIGRP, a 32 bit router ID must be

manually created. It will not automatically create one based on

highest loopback or interface address. The RID that is assigned will

then be used to determine the DR and BDR on a segment (highest

wins).

• OSPFv3 has dropped its native authentication options. Instead, it

relies on the underlying authentications built into IPv6, like IPSec.

Configuration

The configuration is now done on each individual interface. The

following is an example configuration:

To Configure OSPFv3:


!

R2(config)# ipv6 router ospf 100

R2(config-rtr)# router-id 10.10.10.1

R2(config-rtr)# area 1 stub no-summary


!

R2(config)# interface gig1/1

R2(config-if)# ipv6 address 2003:2:1:2::1/64

R2(config-if)# ipv6 ospf 100 area 0

!

R2(config)# interface gig1/2

R2(config-if)# ipv6 address 2003:2:1:A::1/64

R2(config-if)# ipv6 ospf 100 area 1

R2(config-if)# ipv6 ospf priority 30

84 | P a g e

MP-BGP MP-BGP, or multiple protocol BGP, was outlined in RFC 2858 and includes extensions

to the original BGP standard that allows support for other protocols – one of which is

IPv6! The command address-family was added to specify which new protocol

functionality is being configured and is used when applying IPv6 addressing.

Like EIGRP and OSPFv3, an IPv4 address must be configured as a router ID. The

BGP configuration is not done at the interface level, it still is done in router

configuration mode. The major difference is that neighbors must be first defined

under router BGP configuration mode and then “activated” under IPv6 address-family

mode submode. Networks and other parameters are also configured under IPv6

address-family mode submode.

Migrating to IPv6 Three options exist for transitioning from IPv4 to IPv6: dual stack, tunneling, or NAT:

• Dual Stack - this involves running IPv4 alongside IPv6 on the same system.

• Tunneling - this option allows you to encapsulate IPv6 traffic within an IPv4 header.

• NAT - a new network translation extension, NAT-PT allows IPv6-to4 translation.

To Configure MP-BGP:


!


R3(config-rtr)# router-id 10.10.10.10

R3(config-rtr)# neighbor 2003:76:1:1::10

remote-as 700

R3(config-rtr)# address-family ipv6 unicast

R3(config-rtr-af)# neighbor 2003:76:1:1::10

activate

R3(config-rtr-af)# network 2003:2:2::/48

R3(config-rtr-af)# exit


85 | P a g e

Dual Stack Using a dual-stack transition allows servers, clients, and applications to be slowly moved to IPv6. Both protocols can run concurrently, neither

communicating with the other. If both IPv4 and IPv6 are running on a server for example, IPv6 will be used.

Dual-stacking IPv4 alongside IPv6 on systems works well, but it requires most of your infrastructure to support IPv6. In many cases, the network core

does not support IPv6 or it has not been implemented. IPv6 tunnels solve this problem by allowing IPv6 islands to exist and bridging them over IPv4

systems.

Because IPv6 tunnels provide virtual IPv6 connectivity through an IPv4 transport, it does not matter what specific IPv4 transport is used. The only

requirement is that there is end-to-end IPv4 connectivity between both ends.

Manual Tunnels

The tunnels discussed here are from one router to another. The source router (RouterA) encapsulates the IPv6 traffic in IPv4 headers, and then forwards

it to the other end of the tunnel (Router B). Router B then decapsulates the packets and forwards them on to their destination using native IPv6.

Manual IPv6 tunnels are easy to configure using the tunnel mode ipv6ip command. Using the Router A/B example above, the configuration on Router

A would look something like this:

Configuration Example:

R1# config t


R1(config)# ipv6 cef

!

R1(config)# interface serial1/0/1

R1(config-if)# ip address 192.168.1.1 255.255.255.0

R1(config-if)# ipv6 address 2001:1:3:1:1/64

86 | P a g e

GRE Tunnels

First, GRE tunnels are the default tunnel method on Cisco routers. GRE tunnels are very flexible and work over most protocols.

The configuration is exactly the same as the manual configuration example above, but you do not have to specify the tunnel mode. Also, routing

protocols can be enabled on GRE tunnel interfaces just as if they were physical interfaces.

6to4 Tunnels

• 6to4 tunnels are similar to the manual tunnel, but set up the tunnel dynamically.

• 6to4 tunnels use 2002::/16 IPv6 addresses in front of the 32 bit IPv4 address of the edge router – creating a 48 bit prefix. Each router on both sides

of the tunnel needs a route to its peer. They only support static and BGP routes, so be careful.

• Configure the tunnel as if it was a manual tunnel, using the IPv4 address as the source, but don’t enter a destination. The tunnel requires an IPv6

address using the method just described. Finally, use the command tunnel mode ipv6ip 6to4.

NAT Translation is a unique solution because it allows IPv4 devices to communicate with IPv6 devices without the dual stack requirement. NAT-PT allows

bidirectional translation services.

RouterA(config)# interface tunnel0

RouterA(config-if)# ipv6 address 2001:2:0:7::/64

RouterA(config-if)# tunnel source 10.1.1.1

RouterA(config-if)# tunnel destination 10.3.3.1

RouterA(config-if)# tunnel mode ipv6ip

RouterA(config-if)# exit

87 | P a g e

R1(config)# ipv6 nat v4v6 source ipv4_address ipv6_address

1. To enable NAT-PT IPv4 to IPv6 translation on a router, the first step is to use the ipv6 nat command on each interface participating in the

translation.

2. The second step is to define at least one NAT-PT prefix. Only traffic matching the prefix will be translated. To apply it globally on the router, enter

ipv6 nat prefix/prefix_length in global configuration mode. To apply it to traffic on a specific interface, enter ipv6 nat prefix/prefix_length in

interface configuration submode.

3. Define the address mappings (either static or dynamic) using the options discussed below.

Static NAT-PT

For an IPv6 to IPv4 static mapping: For an IPv4 to IPv6 static mapping:

Dynamic NAT-PT

There are many ways to implement dynamic NAT using IPv6, but at its most basic level a pool of addresses is created and the router temporarily assigns

them to hosts as needed.

For an IPv4 to IPv6 dynamic mapping:

For an IPv6 to IPv4 dynamic mapping:

R1(config)# ipv6 nat v6v4 source ipv6_address ipv4_address

R1(config)# ipv6 nat v4v6 pool name begining_ipv6 ending_ipv6 prefix-length prefix-length

R1(config)# ipv6 nat v4v6 source list (access-list_number | name) pool name

R1(config)# ipv6 nat v6v4 pool name begining_ipv4 ending_ipv4 prefix-length prefix-length

R1(config)# ipv6 nat v6v4 source list (access-list_number | name) pool name

88 | P a g e

Chapter 8:

Simulation Scenarios

Cisco 642 902

89 | P a g e

EIGRP/OSPF Redistribution Simulation Example:

PROBLEM • In this scenario, routers R2 and R3 need to be configured for redistribution between

their respective EIGRP and OSPF Autonomous Systems.

• Only R2 and R3 can be configured and the traffic path from R1 to the 10.1.1.0

network should use the links with the greatest bandwidth.

• When completed, router R1 should be able to ping a host in the 10.1.1.0/24 network.

90 | P a g e

SOLUTION First we need to find out 5 parameters (Bandwidth, Delay, Reliability, Load, MTU) of the s1/1 interface (the interface of R2 connected to R4) for

redistribution:

Now write down these 5 parameters, notice that we have to divide the Delay by 10 because its metric unit is in tens of microseconds.

For example, if we get:

Bandwidth=1544 Kbit

Delay=20000 us

Reliability=255

Load=1

MTU=1500 bytes

…then we would redistribute as follows:

R2# show interface s1/1

R2#config terminal


R2(config-router)# redistribute eigrp 200 metric-type 1 subnets


R2(config-router)# router eigrp 200

R2(config-router)# distance eigrp 90 109

R2(config-router)# redistribute ospf 1 metric 1544 2000 255 1 1500

91 | P a g e

Notice in the configuration above the "distance eigrp 90 109" command on R2. This drops the external EIGRP administrative distance from 170 to 109

(one less than OSPF’s AD value of 110) coming in from R3 so that it will be more trustworthy than the OSPF link between R2 and R4.

For R3 we use the show interface fa1/1 to get the same 5 parameters.

For example we get Bandwidth=10000 Kbit, Delay=1000 us, Reliability=255, Load=1, MTU=1500 bytes.

Now let’s configure it the same way we did R2:

Verification

Perform a “show ip route” on R1 to see the 10.1.1.0/24 network (the network behind R4) in the routing table.

Next, ping from R1 to the network to validate the connectivity.

Finally perform a traceroute on R1 to the fa1/1 interface of R1 to make sure the traffic is going form R1-R2-R3-R4. This fulfills the “highest bandwidth”

requirement – using the Fast Ethernet links instead of the Serial connection.

R3# show interface fa1/1

R3#config terminal

R3(config)#router ospf 1

R3(config-router)#redistribute eigrp 200 metric-type 1 subnets

R3(config)#exit

R3(config-router)#router eigrp 200

R3(config-router)#redistribute ospf 1 metric 10000 100 255 1 1500

92 | P a g e

IPv6 OSPF Virtual Link Simulation Example:

PROBLEM • In this scenario, two organizations have merged and need to

connect their core routed networks. Luckily, both have already

implemented IPv6 routing using OSPF, but their area numbers

do not fit together well.

• You have been tasked with getting their core OSPF routers up

and running using their current area configuration until a full

redesign can be performed.

• Currently, R4’s loopback address cannot be seen in R1’s routing

table (and vice-versa).

93 | P a g e

SOLUTION You should know by now that in OSPF, all areas must connect back to the backbone area (area 0). In this case, that isn’t an option because the

directions specifically ask us not to change the current area assignments.

The solution? A virtual link!

We can configure area 1 as a transit area for area 2 using the area virtual-link command.

(Notice that we have to use neighbor router-id 3.3.3.3, not R2′s router-id 2.2.2.2)

Now onto R3:

That’s all there is to it!

Verification: To verify that R1 has a route to R4’s loopback interface, use the show ipv6 route command on R1.

R2> enable

R2# configure terminal


R2(config-rtr)# area 1 virtual-link 3.3.3.3

R3> enable

R3# configure terminal


R3(config-rtr)# area 1 virtual-link 2.2.2.2

94 | P a g e

OSPF Simulation Example:

PROBLEM • Sharky’s Surf ‘n Turf is a fast-growing corporate seafood establishment and needs your help. A new HQ office was recently constructed with

connectivity provided by router R1. Your task is to configure and verify connectivity between the current HQ head-end router (R2) and the new

location (R1).

• The physical cabling between R1 and R2 has been completed, but the configuration of OSPF Area 1 needs to be completed to include ONLY R1

s1/0 and R2 s1/1. The mask should be configured to allow only the two interfaces to participate in the OSPF area.

• Also, Area 1 should be configured in a way so that it does not receive any inter-area or external routes (except default routes). The interfaces

have already been configured with the IP addresses shown in the diagram.

95 | P a g e

SOLUTION 1. Let’s start with R1.

There are two primary considerations in this scenario. First, Area 1 must be configured to include only R1 and R2’s interfaces. The network that

the circuit is using is /30, meaning we need to use a wildcard mask for the OSPF area that reflects that /30 address range. 0.0.0.3 will do just

that.

The second major consideration for this scenario is that Area 1 should be configured in a way so that it does not receive any inter-area or external

routes (except default routes). That’s code for a totally stubby area. In this case, R1 needs to have the stub command applied.

2. Moving on to R2.

The same network wildcard must be applied to R2:

Notice the area 1 stub no-summary command. R2 is the ABR, so if we want Area 1 to be a totally stubby area then we need to use the no –summary command here.

That’s it! Fairly simple, but VERY important practice for

the exam.

R1#config terminal



R1(config-router)# area 1 stub


R1#config terminal



R1(config-router)# area 1 stub no-summary


96 | P a g e

Policy-Based Routing Simulation Example:

PROBLEM

• You’ve been tasked with providing a routing policy solution to a

new web-startup company. They have heavy outbound HTTP

traffic loads and want to use a dedicated frame relay circuit to

carry it.

• Configure router PBR in such a way that all HTTP traffic traverses

the frame relay link to ISP A if the link is up. All other traffic can

go through either link. Only router PBR can be configured and

due to network policies, static routes and default routes are not

allowed.

97 | P a g e

SOLUTION 1. First we need to create an access list that defines the web traffic.

A source of “any” is used to capture all EIGRP network sources.

2. Now we create a route map that sets the next-hop for the web traffic.

Notice that the first statement sets the next-hop address for the HTTP traffic and the following route map line (20) allows any other traffic

through unmodified. If line 20 wasn’t used, the implicit deny would drop any non-web traffic.

3. Last step is to apply the route map to the internal-facing interface on router PBR.

PBR(config)# access-list 101 permit tcp any any eq www

PBR(config)# route-map PBR permit 10

PBR(config-route-map)# match ip address 101

PBR(config-route-map)# set ip next-hop 10.1.1.1

PBR(config-route-map)# exit

PBR(config)# route-map PBR permit 20

PBR(config-route-map)# exit

PBR(config)# int fa0/1

PBR(config-if)# ip policy route-map PBR

PBR(config-if)# exit

PBR(config)# exit

98 | P a g e

4. Verification

On a host in the internal EIGRP network, generate HTTP traffic destined for the internet.

Next, use the show route-map command to verify that packets are being matched against the new filter.

You should see something like “Policy routing matches: 12 packets…” in the output if your configuration is correct.

PBR# show route-map

Documents

ROUTE Exam Guide v3.3