Www.ciscopress.com Routers and Routing Basics CCNA 2 Chapter 7

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Routers and Routing Basics CCNA 2 Chapter 7

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Distance Vector Routing Protocols

Using Distance Vector Routing ProtocolsReview of Distance Vector Operation in a Stable Network Route PoisoningProblem: Counting to InfinityLoop-Prevention FeaturesSummarizing Loop Avoidance

Routing Information ProtocolConfiguring RIP Versions 1 and 2RIP Verification and TroubleshootingChoosing the Best Route Among the Possible RoutesIntegrating Static Routes with RIPClassful and Classless Routing Protocols, Routing, and Addressing

Summary

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Routing Loops – the Fee for Simplicity

In 1980s a typical WAN link to a remote site might have been only 56 kbps

As a result, the designers of the first distance vector protocols had to keep them simple

The simplicity of distance vector protocols introduced the possibility of routing loops (same single packet ends up back at the same routers over and over again)

The looping packets could easily congest the network and make it unusable

Routing loops must be avoided as much as possible

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Distance Vector Operation in a Stable Network

1. R2 considers itself to have a 0-hop route for subnet 172.30.22.0/24, so in the routing update sent by R2, R2 advertises a metric 1 (hop count 1) route.

2. R1 receives the update, and because R1 has learned of no other possible routes to 172.30.22.0, this route must be R1’s best route to the subnet.

3. R1 adds the subnet to its routing table, listing it as a RIP-learned route.

4. For the learned route, R1 uses an outgoing interface of S0/0, because R1 received R2’s routing update on R1’s S0/0 interface.

5. For the learned route, R1 uses a next-hop router of 172.30.1.2, because R1 learned the route from a RIP update whose source IP address was 172.30.1

At the end of this process, R1 has learned a new route. The rest of the RIP-learned routes in this example follow the same process.

Normal Steady-State RIP Operations

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Distance Vector Operation in a Stable Network (Continued)

Metric—RIP uses hop count for the metric. RIP routers add 1 to the metric before advertising the route.

Periodic—The hourglass icons represent the fact that the updates repeat on a regular cycle. RIP uses a 30-second update interval by default.

Full updates—The routers send full updates, every time, instead of just sending new or changed routing information. (The term partial update refers to routing updates that include only changed information.)

Full updates limited by split horizon rules—The routing protocol omits some routes from the periodic full updates due to split horizon rules.

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Route Poisoning

When a route fails, distance vector routing protocols risk causing routing loops until every router in the internetwork knows and believes that the original route has failed.

As a result, distance vector protocols need to have a way to

specifically identify which routes have failed.

Distance vector protocols spread the bad news about a route failure by poisoning the route. Route poisoning refers to the practice of advertising a route, but with a

special metric value called infinity.

Simply put, routers consider routes advertised with an infinite metric to have failed. Each distance vector routing protocol uses the concept of an actual metric value that represents infinity.

RIP defines infinity as 16.

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Route Poisoning (Continued)

1. R2’s FA0/1 interface fails.

2. R2 removes its connected route for 172.30.22.0/24 from its routing table.

3. R2 advertises 172.30.22.0 with an infinite metric, which for RIP is metric 16.

4. R1 keeps the route in its routing table, with an infinite metric, as part of the loop-avoidance process.

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Counting to Infinity

Distance vector routing protocols risk causing routing loops between the time at which the first router realizes a route has failed until all the routers know that the route has failed.

That problem, called counting to infinity, causes two other related problems:

1. Packets may loop around the internetwork while the routers count to infinity, with the bandwidth consumed by the looping packets crippling an internetwork.

2. The counting-to-infinity process may take several minutes, meaning that the looping could cause users to believe that the network has failed.

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Counting to Infinity (Continued)

1. R2’s FA0/1 interface fails, so R2 removes its connected route for 172.30.22.0/24 from it routing table.

2. R2 sends a poisoned route advertisement (metric 16 for RIP) to R1, but at about the same time, R1’s periodic update timer expires, so R1 sends its regular update, including an advertisement of 172.30.22.0, metric 2.

3. R2 hears about the metric 2 route to reach 172.30.22.0 from R1. Because R2 no longer has a route for subnet 172.30.22.0, R2 adds the two-hop route to its routing table, next-hop router R1.

4. At about the same time as Step 3, R1 receives the update from R2, telling R1 that its former route to 172.30.22.0, through R1, has failed. As a result, R1 changes its routing table to list a metric of 16 for the route to 172.30.22.0.

R2 Incorrectly Believes R1 has a Route to 172.16.22.0/24

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Counting to Infinity (Continued)

1. Both R1’s and R2’s update timers expire at about the same time. R1 advertises a poison (metric 16) route, and R2 advertises a metric 3 route. (Remember, RIP routers add 1 to themetric before advertising the route.)

2. R2 receives R1’s update, so R2 changes its route for 172.30.22.0 to use a metric of 16.

3. At about the same time as Step 2, R1 receives R2’s update, so R1 changes its route for 172.30.22.0 to use a metric of 3.

R1 and R2 Count to Infinity


Loop-Prevention Features – Split Horizon

Split horizon is defined as follows:

In routing updates sent out interface X,

do not include routing information about routes

that refer to interface X as the outgoing interface.


Loop-Prevention Features – Split Horizon (Continued)

1. R1 sends its normal periodic full update, which, due to split horizon rules, includes only one route.

2. R2 sends its normal periodic full update, which, due to split horizon rules, includes only two routes.


4. R2 removes its connected route for 172.30.22.0/24 from its routing table.

5. R2 advertises 172.30.22.0 with an infinite metric, which for RIP is metric 16.

6. R1 temporarily keeps the route for 172.30.22.0 in its routing table, later removing the route from the routing table.

7. In its next regular update, R1, due to split horizon, still does not advertise the route for172.30.22.0.

The Effects of Split Horizon Without Poison Reverse


Poison Reverse and Triggered Updates

Distance vector protocols can attack the counting-to-infinity problem when reacting to failed routes by ensuring that every router learns that the route has failed, through every means possible, as quickly as possible:

■ Triggered update — When a route fails, do not wait for the next periodic update. Instead, send an immediate triggered update listing the poisoned route.

■ Poison reverse — When learning of a failed route, suspend split horizon rules for that route, and advertise a poisoned route.


Poison Reverse and Triggered Updates(Continued)


2. R2 immediately sends a triggered partial update with only the changed information—inthis case, a poison route for 172.30.22.0.

3. R1 responds by changing its routing table and sending back an immediate (triggered) partial update, listing only 172.30.22.0 with an infinite metric (metric 16). This is a poisonreverse route.

4. On R2’s next regular periodic update, R2 advertises all the typical routes, including thepoison route for 172.30.22.0, for a time.

5. On R1’s next regular periodic update, R1 advertises all the typical routes, including thepoison reverse route for 172.30.22.0, for a time.

R2 Sending a Triggered Update, with R1 Advertising a Poison Reverse Route


Loops in Redundant Networks

Split horizon prevents the counting-to-infinity problem from

occurring between two routers.

However, with redundant paths in an internetwork, which is true of most internetworks today, split horizon alone does not always prevent counting to infinity.


Loops in Redundant Networks (Continued)

1. R2 advertises a metric 1 route for 172.30.22.0 in its updates to both R1 and R3.

2. R1 advertises a metric 2 route for 172.30.22.0 to R3, while R3 advertises a metric 2 route for 172.30.22.0 to R2.

3. Both R1 and R3 add the metric 1 route, learned directly from R2, to their routing tables, and ignore the two-hop routes they learn from each other. For example, R1 places a route 172.30.22.0, using outgoing interface S0/0, next-hop router 172.30.1.2 (R2), in its routing table.

Periodic Updates in a Stable Triangle Internetwork




2. R2 immediately sends triggered partial updates poisoning the route for 172.30.22.0. R2 sends the updates out all still-working interfaces.

3. R3 receives R2’s triggered update that poisons the route for 172.30.22.0, so R3 updates its routing table to list metric 16 for this route.

4. Before the update described in Step 2 arrives at R1, R1 sends its normal periodic update to R3, listing 172.30.22.0, metric 2, as normal. (Figure omits some of what would be in R1’s periodic update to reduce clutter.)

Counting to Infinity in a Redundant Internetwork, Part 1

5. R1 receives R2’s triggered update (described at Step 2) that poisons the route for 172.30.22.0, so R1 updates it routing table to list metric 16 for this route.

6. R3 receives the periodic update sent by R1 (described at Step 4), listing a metric 2 route for 172.30.22.0. As a result, R3 updates its routing table to list a metric 2 route, through R1 as the next-hop router, with outgoing interface S0/0.At this point, R3 has an incorrect metric 2 route for 172.30.22.0, pointing back to R1.



Counting to Infinity in a Redundant Internetwork, Part 2 7. R1 sends its next periodic update to R3,

with poisoned route 172.30.22.0, metric 16.

8. Before the update described in Step 7 arrives at R3, R3 sends its next periodic update toR2, listing a metric 3 route for 172.30.22.0.

9. R3 receives R1’s periodic update from R1 (as described in Step 7), and R3 changes its route for 172.30.22.0 to list an infinite metric.

10. R2 receives R3’s periodic update (as described in Step 8), so R2 adds a metric 3 route for 172.30.22.0 to its routing table, listing R3 as the next-hop router, with outgoing interfaceS0/1/


The Holddown Process and Holddown Timer

Distance vector protocols use holddown to specifically

attack the loops created by the counting-to-infinity

problems that occur in redundant internetworks

The term holddown gives a hint as to its meaning:

After the route is considered to be down, hold the route in a down state for a while to give the routers time to make sure every router knows that the route has failed.

The holddown process tells a router to ignore new information about

the failed route, for a time period called the holddown time, as counted

using the holddown timer.


Using Holddown to Prevent Counting to Infinity


2. R2 immediately sends triggered partial updates, poisoning the route for 172.30.22.0. R2 sends the updates out all still-working interfaces.

3. R3 receives R2’s triggered update that poisons the route for 172.30.22.0, so R3 updates its routing table to list metric 16 for this route. R3 also puts the route for 172.30.22.0 in holddown and starts the holddown timer (180 seconds by default with RIP) for the route.

4. Before the update described in Step 2 arrives at R1, R1 sends its normal periodic update toR3, listing 172.30.22.0, metric 2, as normal. (Note that Figure 7-10 omits some details in R1’s periodic update to reduce clutter.)

5. R1 receives R2’s triggered update (described in Step 2) that poisons the route for172.30.22.0, so R1 updates its routing table to list metric 16 for this route.

6. R3 receives the update from R1 (Step 4), listing a metric 2 route for 172.30.22.0. Because R3 has placed this route in a holddown state, and this new metric 2 route was learned from a different router (R1) than the original router (R2), R3 ignores thenew routing information.


Summarizing Loop Avoidance

During periods of stability, routers send periodic full updates. The updates list all known routes except the routes omitted due to split

horizon rules.

When changes occur that cause a route to fail, routers react by sending triggered partial updates with poisoned routes. Routers also suspend split horizon rules for that route

advertising a poison reverse route back toward the router from which the failed route was learned

All routers place a route in holddown state and start a holddown timer for that route after learning that the route has failed. The router ignores all new information about that route until the

holddown timer expires, unless that information comes from the same router that originally advertised the good route to that subnet.


Distance Vector Loop Avoidance Terminology


Routing Information Protocol

The first IP networks used RIP Version 1 (V1) because it was the first and only IP routing protocol early in the history of TCP/IP.

As time went on, routers became more affordable, with

faster CPUs, more memory, and faster links, all of which allowed the development of more advanced routing algorithms and routing protocols, such as OSPF and EIGRP.

Around the same time, other developers enhanced the RIP protocol standard, calling the new standard RIP Version 2 (V2).

RIP V2 does not completely change RIP V1, but rather adds some advanced features.


Comparing RIP Version 1 and 2 Features


Configuring RIP V1

RIP V1 configuration requires two configuration commands:

- router rip

- network classful-network-number

The router rip command moves the user from global

configuration mode to RIP configuration mode, and the

network command tells the router on which interfaces to

start using RIP.


Configuring RIP V1 (Continued)

Configuring RIP on All Interfaces on R1


Configuring RIP V1 (Continued)

When a router’s RIP configuration matches an interface,

Cisco IOS starts the following process:

1. Sends RIP updates out the interface.

2. Listens for RIP updates coming in that interface from some other router.

3. Advertises the subnet attached to the interface.


Configuring RIP V2

1. To configure RIP V2 in internetworks that use RIP V2 only, simply add the version 2 command under router rip.

2. After they are configured, the routers send only V2 updates

And process only received V2 updates. 3. At that point, the core features of RIP V2, such as sending

masks in routing updates occur. 4. Optional RIP V2 features, such as authentication, this

requires additional configuration.


Using Both RIP V1 and V2

1. In some cases, an internetwork may need to use both RIP versions.

(Partial migrating from RIP V1 to RIP V2, some business or company organizational reason to use both versions, etc.)

2. Regardless of the reasons, to support both versions in the same internetwork, one or more routers need to use both versions at the same time.


RIP Version Migration: Speaking Both Versions

1. Configure R1 for RIP V1 (by omitting the version command) and then configure interface S0/0 to send and receive RIP V2 updates

2. Configure R1 for RIP V2 (by including the version 2 command) and then configure interface S0/1 to send and receive RIP V1 updates


Configuring RIP Version 2 on an Interface

R1 enables RIP V2 on interface S0/0 by using the ip rip send version 2 and ip rip receive version 2 interface subcommands.

So, R1 sends and receives only RIP V2 updates on the right of Figure and defaults to sending and receiving RIP V1 updates on the left.


Design Options Impacted by the RIP V2

The use of RIP V2 instead of RIP V1 allows the use of two powerful network design options.

1. V2 allows for the use of VLSM. VLSM gives the engineer much moreflexibility when choosing which subnets to use and how many hosts to put into each subnet.

2. RIP V2 also allows a design choice called a discontiguous network. A discontiguous network occurs when at least one pair of subnets of the same classful network are separated by subnets of a different classful network. RIP V1 does not support discontiguous networks; RIP V2 supports them if all the routers have been configured with the RIP no auto-summary subcommand.


Discontiguous Network 172.30.0.0


Other RIP Configuration Options

RIP has several optional configuration settings as well:

Adjust timers, such as the holddown and update timers Enable or disable split horizon per interface Explicitly configure RIP neighbors to support certain

types of WAN connections Disable the sending of RIP updates on an interface

(using the passive-interface command), while still receiving RIP updates

Filter the contents of RIP updates


RIP Timers

RIP uses several timers: 1. Update timer2. Holddown timer

RIP uses the concept of an invalid timer and a flush timer. (The flush timer determines when a router removes a route from the routing table after the route has been poisoned.)

All of these timers can be reset with the following command, which is configured as a subcommand under router rip:

timers basic update invalid holddown flush

You might consider lowering the holddown timer to speed convergence.


Disabling Split Horizon

Split horizon helps prevent loops by avoiding the counting-to-infinity

problem. Cisco IOS enables split horizon on all interfaces

(except serial interfaces that are configured with some of Frame Relay

options). However, you can disable split horizon, per interface using

interface subcommand:

no ip split-horizon

For example, to disable split horizon on interface S0/0, the engineer

would enter configuration mode, type the interface S0/0 command,

and then use the no ip split-horizon command.


Configuring Neighbors

RIP V1 sends its update messages to IP broadcast address 255.255.255.255. RIP V2 Improves the update process by sending its update messages to the 224.0.0.9 multicast IP address.

By using multicasts, only RIP-speaking routers should process RIP updates, reducing the overhead on the other hosts on a LAN. However, some WAN data links may not support the sending of data-link broadcasts or multicasts.

In those cases, RIP must send its updates using IP and datalink unicast addresses.

To do so, a RIP router must define the neighboring router’s unicast IP address using the neighbor command under router rip.


Enabling the passive-interface Command

After RIP is configured, it may be useful to then stop sending RIP

updates on the interface.

To do so, the configuration must still match the interface with a

network command, and then the router must be told to stop sending

updates with the passive-interface interface subcommand under

router rip.

The passive-interface command tells RIP to stop sending RIP updates

out the listed interface.


Route Redistribution

R2, which uses only OSPF, has no need to receive R1’s RIP updates. So, R1 has used the passive-interface command, meaning that R1 no longer sends RIP updates out its S0/0 interface into the OSPF part of the internetwork.


Filtering Routes

Routers can use route filtering to filter the routes sent and

received in RIP updates.

Route filtering allows an engineer to limit which routers

learn which routes.

For example, if a particular subnet should be protected for security

reasons, and only certain groups of people should be able to

communicate with the hosts in that subnet, the engineer could filter

routes.


Verifying RIP Operations Using show Commands

The following four show commands provide the most

useful information for examining how RIP is working in a

router:

- show ip protocols

- show ip route

- show ip interface brief

- show ip rip database


R1: Sample RIP show ip protocols Command


Sample RIP show Commands on R1


Troubleshooting RIP Operations Using the debug Command

Cisco IOS supports a very important troubleshooting command called the debug command.The debug command has many options, including options related to RIP. Regardless of what options are added to the debug command, this command tells the router to do the following:

- Monitor some internal process (for example, RIP updates that are sent and received)- When something happens related to that process, generate log messages- Keep generating log messages until someone disables the debug using the no debug command


R1: Messages Generated by the debug ip rip Command


Load Balancing over Multiple Equal-Cost Routes

When a router discovers multiple equal-cost routes to the same subnet,

using a single routing protocol, the routing protocol can add multiple of

those routes to the routing table.

All the IGPs on Cisco routers use the following (default) rules when

considering multiple equal-cost routes:

- By default, add up to four equal-cost (equal-metric) routes for the same subnet to the routing table at the same time.

- The number of concurrent equal-cost routes can be changed by using the maximum-paths number subcommand, to a value between 1 and 6.


Load Balancing over Multiple Equal-Cost Routes (Continued)

When the IP routing table lists multiple routes to the same destination, the IP routing process then needs to choose how to load-balance the traffic over the multiple routes. The following two options based on the internal routing process used by the router:

- Process switching — The slowest and highest-overhead option for how IOS forwards packets. However, with process switching, load balancing occurs per packet, with each successive packet going to the destination subnet using a different route.

- Fast switching — The next fastest option, with less overhead, for how IOS forwards packets. However, when using fast switching, the router balances traffic per destination IP address.


Equal-Cost Load Balancing

R1: Messages Generated by the show ip route Command


Choosing Routes Based on Administrative Distance

In some cases, one router may need to use multiple routing protocols.

Because each routing protocol uses a different metric, a router cannot

use the metric to determine which route is the best route.

Routers determine the best route in these cases by choosing the route

with the lowest administrative distance.

The administrative distance is a number assigned to all the possible

sources of routing information — routing protocols and static routes

included.


Default Administrative Distances in Cisco IOS


Floating Static Routes

A floating static route is a static route that the engineer wants to be used some of the time. The term floating comes from the idea that the static route leaves the routing table under some conditions and comes back into the routing table under other conditions.

Floating static routes can be very useful for dial backup, using the following logic:

- When a permanent WAN connection is up, the router should ignore the static route and instead use the routes learned by the routing protocol. These routes will forward packets out the permanent WAN connection.

- When the permanent WAN connection is down, use the statically defined route that sends traffic over the dial backup link.


Advertising Default Routes with RIP

In some cases, it makes sense to distribute a default route throughout an internetwork.

1. All routers in the enterprise internetwork learn about all subnets of Class B network 130.1.0.0 via RIP.

2. Router R-core defines a static default route pointing to the Internet.

3. Router R-core advertises a default route to the rest of the routers in the enterprise.


Classless and Classful Routing Protocols

The term classless routing protocol refers to a set of routing protocols that provide a particular set of functions.

Classless routing protocols perform the following functions:

- Send subnet mask information in routing updates- Support variable-length subnet mask (VLSM) because of the

inclusion of the mask in routing updates- Support designs that include discontiguous networks

A classful routing protocol, by definition, does not send mask information. As a result, it does not support VLSM, nor does it support discontiguous networks.


Classless and Classful Routing

The terms classful routing and classless routing refer to how each router uses its default route, assuming the router has a default route.

- Classless routing—If a packet’s destination IP address does not match a more specific route in the IP routing table, forward the packet based on the default route.

- Classful routing—If a packet’s destination IP address does not match a more specific route in the IP routing table, forward the packet based on the default route, but only if the routing table does not contain any subnets of that packet’s classful IP network.


Classless and Classful Addressing

The terms classless addressing and classful addressing refer to two methods of analyzing the structure of IP addresses. Classful addressing means that, when analyzing IP addresses, the addresses are considered to have a one-, two-, or three-octet network part, with the remainder of the addresses being the host part.

Classless addressing ignores Class A, B, and C rules, treating each IP address as having only two parts: a subnet part and a host part. In a classless address, the subnet part (also called the prefix) contains what would have been the combined network and subnet parts with classfuladdressing.


Classless and Classful Addressing Compared


Summary

Distance vector routing protocols use a wide variety of loop-avoidance features.


Summary (Continued)


Summary (Continued)

Distance vector algorithms call for each router to send its entire routing table, on a periodic basis, to each of its adjacent neighbors. Therouting tables include information about each network or subnet, along with the metric associated with each network or subnet.

RIP Version 2 added many features to RIP Version 1. These enhancements include an authentication mechanism, support of VLSM, and support of discontiguous networks.

The two most common commands used to verify that RIP is properly

configured are the show ip route and show ip protocols commands.


Summary (End)

The passive-interface command prevents routers from sending routing updates through a router interface.

RIP supports the function of adding multiple equal-cost routes to the same subnet—up to four routes by default, and up to six possible.

Classless routing, as enabled with the ip classless global configuration command, means that a router always uses its default route (assuming one exists) if a packet’s destination address doesnot match another route.

With classful routing enabled (with the no ip classless global command), the router may discard some packets even if a default route exists.

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Www.ciscopress.com Routers and Routing Basics CCNA 2 Chapter 7