Ch. 6 – Routing and Routing Protocols

CCNA 1 version 3.0

Rick Graziani

Cabrillo College

Rick Graziani graziani@cabrillo.edu 2

Overview

• Explain the significance of static routing

• Configure static and default routes

• Verify and troubleshoot static and default routes

• Identify the classes of routing protocols

• Identify distance vector routing protocols

• Identify link-state routing protocols

• Describe the basic characteristics of common routing protocols

• Identify interior gateway protocols

• Identify exterior gateway protocols

• Enable Routing Information Protocol (RIP) on a router

The Routing Table

• Before we begin lets see lets discuss the routing table and directly connected routes.

• This is information which is not in the curriculum, but will give you a better understanding of what is taking place.

The Routing Table prior to any interface configuration• The command to view the IP Routing table is: (priviledge or user mode)

Router# show ip route• Currently, no routes in the routing table.

Directly Connected Networks and the IP Routing Table

RTA#show ip route

Codes: C - connected, S - static, I - IGRP, 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, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default

U - per-user static route, o - ODR

Gateway of last resort is not set

172.16.0.0/16 192.168.1.0/24

.1 .1.2 .2

RTA RTB RTC192.168.2.0/24

10.0.0.0/8

s0 s0 s1s1e0 e0

• Adding an ip address/mask to an interface tells the router that it is a member, “Directly Connected” to that network – just like when a host computer is configured with an ip address/mask.

• Notice the route is shown with the subnet mask and the “exit-interface.”

• Don’t forget the “no shutdown”

• Don’t forget the interface must be in “up” and “up”

RTA(config)#inter e 0

RTA(config-if)#ip add 192.168.2.1 255.255.255.0

RTA(config-if)#no shutdown

RTA#show ip route

Codes: C - connected,.. <Other codes and gateway information omitted>

C 192.168.2.0/24 is directly connected, Ethernet0

172.16.0.0/16 192.168.1.0/24

.1 .1.2 .2

RTA RTB RTC192.168.2.0/24

10.0.0.0/8

s0 s0 s1s1e0 e0

Viewing the Routing Table Process• Use the “debug ip routing” command to view the Cisco IOS routing table process of

adding a directly connected network to the routing table.

• When finished, be sure to use “undebug all”

• Debug commands are used to view detailed information about Cisco IOS processes – more later.

RTA# debug ip routing

RTA(config-if)#ip add 192.168.2.1 255.255.255.0

RTA(config-if)#no shutdown

00:28:56: RT: add 192.168.2.0/24 via 0.0.0.0, connected metric [0/0]

00:28:56: RT: interface Ethernet0 added to routing table

RTA#show ip route

RTA# undebug all

Viewing the Routing Table Process• Directly connected routes will also be removed if the link goes down.

• Directly connected routes will only be in the routing table if, it is not administratively down, the line is “up” and protocol is “up”

• For serial interfaces, don’t forget the “clock rate” command on the router with the DCE cable – neither interface will be “up” and “up” until both ends are configured correctly.

RTA# debug ip routing

RTA(config-if)#shutdown

00:34:38: RT: interface Ethernet0 removed from routing table

00:34:38: RT: del 192.168.2.0 via 0.0.0.0, connected metric [0/0]

00:34:38: RT: delete network route to 192.168.2.0

RTA#show ip route

RTA# undebug all

RTA#show ip route

C 172.16.0.0/16 is directly connected, Serial0

RTB#show ip route

RTC#show ip route

172.16.0.0/16 192.168.1.0/24

.1 .1.2 .2

RTA RTB RTC192.168.2.0/24

10.0.0.0/8

s0 s0 s1s1e0 e0

The Routing Tables• Notice that the routers only know about their own directly connected networks.• They are not sharing routing information because we have not configured any static routes or dynamic

routing protocols.

Configuring an interface which has a subnet mask greater than the classful mask• We will discuss this in much more detail later using the presentation – “The Routing Table.”

• For now, notice that when the subnet mask is not a classful mask, but a subnetted /16 mask.

• The routing table information shows the route to the subnetted network

• The mask is shown in the above, “parent” classful network.

RTC(config)#inter e 0

RTC(config-if)#ip add 10.1.0.1 255.255.0.0

RTC#show ip route

10.0.0.0/16 is subnetted, 1 subnets

C 10.1.0.0 is directly connected, Ethernet0

172.16.0.0/16 192.168.1.0/24

.1 .1.2 .2

RTA RTB RTC192.168.2.0/24

10.1.0.0/16

s0 s0 s1s1e0 e0

Routing – Only directly connected hosts (routers)• Routers can only reach networks known about in its own routing table.

RTA#show ip route

RTA#ping 172.16.0.1

RTA#ping 172.16.0.2

RTA#ping 192.168.1.1

RTA#ping 192.168.1.2

RTA#ping 10.1.0.1

172.16.0.0/16 192.168.1.0/24

.1 .1.2 .2

RTA RTB RTC192.168.2.0/24

10.1.0.0/16

s0 s0 s1s1e0 e0

Routing– Routing tables must have the necessary network routes• Question: If RTA can ping RTB’s 172.16.0.2 interface why can’t it ping RTB’s 192.168.1.1

interface? - RTA does not have a route to it in its routing table.

• Question: Would an extended ping from RTA, using the source IP address of 192.168.2.1 be able to ping 172.16.0.1 on RTB? Why or why not? Where does the echo request or echo reply fail?

RTA#show ip route

RTA#ping 172.16.0.1

Sending 5, 100-byte ICMP Echos to 172.16.0.1, timeout is 2 seconds:

Success rate is 100 percent (5/5), round-trip min/avg/max = 56/57/60 ms

RTA#ping 172.16.0.2

172.16.0.0/16 192.168.1.0/24

.1 .1.2 .2

RTA RTB RTC192.168.2.0/24

10.1.0.0/16

s0 s0 s1s1e0 e0

Routing– Routing tables must have the necessary network routes• Question: Would an extended ping from RTA, using the source IP address of 192.168.2.1 be able

to ping 172.16.0.1 on RTB? Why or why not?• The echo request from RTA reaches RTB because RTA has a route to 172.16.0.0/16 in its routing

table.• However, the echo reply from RTB back to RTA fails, because RTB does not have a route for

192.168.2.0/24 in its routing table.

RTA#show ip route

RTB#show ip route

RTA#ping

Protocol [ip]:

Target IP address: 172.16.0.2

Extended commands [n]: y

Source address or interface: 192.168.2.1

Sending 5, 100-byte ICMP Echos to 172.16.0.2, timeout is 2 seconds:

172.16.0.0/16 192.168.1.0/24

.1 .1.2 .2

RTA RTB RTC192.168.2.0/24

10.1.0.0/16

s0 s0 s1s1e0 e0

Routing Table Principles Revisited (Zinin, Cisco IP Routing)• Every router makes its decision alone, based on the information it has in its own routing table.• The fact that one router has certain information in its routing table does not mean that other routers

have the same information.• Routing information about a path from one network to another does not provide routing information

about the reverse, or return path.

RTA#show ip route

RTB#show ip route

RTC#show ip route

C 10.1.0.0 is directly connected, Ethernet0

172.16.0.0/16 192.168.1.0/24

.1 .1.2 .2

RTA RTB RTC192.168.2.0/24

10.1.0.0/16

s0 s0 s1s1e0 e0

Back to the curriculum…

Routing Types

Static Route Operation

Hoboken#show ip route

Codes: C - connected, S - static,

S 172.16.1.0/24 [1/0] is directly connected, Serial0

ip route command

RTR(config)# ip route prefix mask {address | interface} [distance] [tag tag] [permanent]

• prefix IP route prefix for the destination.

• mask Prefix mask for the destination.

• address IP address of the “next hop” that can be used to reach that network.

• interface Network interface to use (exit-interface)

• distance (Optional) An administrative distance.

• tag tag (Optional) Tag value that can be used as a "match" value for controlling redistribution via route maps. (CCNP Advanced Routing)

• Permanent (Optional) Specifies that the route will not be removed, even if the interface shuts down. (CCNP Advanced Routing)

• If the exit interface (gateway) is “down” the static route will not be put in the routing table.

• If the router cannot reach the outgoing interface that is being used in the route, the route will not be installed in the routing table.

• This means if that interface is down, the route will not be placed in the routing table.

Administrative Distance and Metric

• [ administrative distance / routing metric (or cost) ]

• The cost for all static routes is “0”

• The default administrative distance for static routes is “1”

Hoboken#show ip route

S 172.16.1.0/24 [1/0] is directly connected, Serial0

Administrative Distance

• Administrative Distance is the “trustworthiness” of the routing information.• Lower the administrative distance the more trustworthy the information.• If the router hears about a route to the same network from more than one

source it will use the administrative distance to decide which route to put in the routing table.

Examples from the curriculum

• Two choices.

• We will see the differences in a moment.

• The network 0.0.0.0 and mask 0.0.0.0 are known as a “default route”

• Can be written 0.0.0.0/0

• Known as a “quad zero” route”

• More later

Static Routing

• Some extra information on static routing that is not in the curriculum

Static Routing

Router(config)#ip route destination-prefix destination-prefix-mask {address | interface} [distance] [tag tag] [permanent]

Static Routing

Configuring static routes

• Routers do not need to configure static routes for their own directly connected networks.

• We need to configure static routes for networks this router needs to reach.

• We will need to configure static routes for the other routers as well, as “routing information about a path from one network to another does not provide routing information about the reverse, or return path.”

• Convergence – When all the routers in the network (AS) have accurate and consistent information, so that proper routing and packet forwarding can take place.

• Convergence will not happen until all the routers have complete and accurate routing information, meaning we must configure static routes on all the routers before packets will be correctly delivered.

172.16.0.0/16 192.168.1.0/24

.1 .1.2 .2

RTA RTB RTC192.168.2.0/24

10.1.0.0/16

s0 s0 s1s1e0 e0

Basic static route example

• Be sure to use the proper subnet mask!

RTA(config)#ip route 192.168.1.0 255.255.255.0 172.16.0.2

RTA#show ip route

S 192.168.1.0/24 [1/0] via 172.16.0.2

Network/subnet routeIntermediate-Address (usually “next-hop”)

172.16.0.0/16 192.168.1.0/24

.1 .1.2 .2

RTA RTB RTC192.168.2.0/24

10.1.0.0/16

s0 s0 s1s1e0 e0

Static Routing

Basic static route example (continued)• [1/0] – [ Administrative Distance / Metric ]• Administrative Distance – This is the “trustworthiness” of the routing information. The

default administrative distance of static routes is 1.• The Administrative Distance of a directly connected route is 0.• Lower the AD the more trustworthy.• If the router learns about a route to a network from more than one source, it will install the

route with the lower administrative distance in the routing table.

RTA#show ip route

S 192.168.1.0/24 [1/0] via 172.16.0.2

172.16.0.0/16 192.168.1.0/24

.1 .1.2 .2

RTA RTB RTC192.168.2.0/24

10.1.0.0/16

s0 s0 s1s1e0 e0

Static Routing

Basic static route example (continued)• [1/0] – [ Administrative Distance / Metric ]• Metric – This is the “cost” of getting to this route, I.e. how far away this network is.• The lower the cost, the closer the network.• Static routes always show a cost of “0” even if it was configured with the intermediate

address is multiple-hops away.• Much more later.

RTA#show ip route

S 192.168.1.0/24 [1/0] via 172.16.0.2

172.16.0.0/16 192.168.1.0/24

.1 .1.2 .2

RTA RTB RTC192.168.2.0/24

10.1.0.0/16

s0 s0 s1s1e0 e0

Static Routing

Recursive Lookup• The router knows it can get to 192.168.1.0/24 network by forwarding the packets to the

router at the ip address of 172.16.0.2

• How does the router know how to get to the ip address 172.16.0.2?

• It does a recursive lookup – first (1) by looking up the 192.168.1.0/24 network and finding it needs to forward the packet to 172.16.0.2 – the router then (2) looks up the 172.16.0.0 network and sees it can forward it out the interface Serial 0.

RTA#show ip route

S 192.168.1.0/24 [1/0] via 172.16.0.2

172.16.0.0/16 192.168.1.0/24

.1 .1.2 .2

RTA RTB RTC192.168.2.0/24

10.1.0.0/16

s0 s0 s1s1e0 e0

Static Routing

Static Routes and the Routing Table Process• Notice that the static route is entered into the routing table by the routing table process (debug ip

routing) with a metric of 0.

172.16.0.0/16 192.168.1.0/24

.1 .1.2 .2

RTA RTB RTC192.168.2.0/24

10.1.0.0/16

s0 s0 s1s1e0 e0

RTA#debug ip routing

IP routing debugging is on

RTA#conf t

Enter configuration commands, one per line. End with CNTL/Z.

05:53:48: RT: add 192.168.1.0/24 via 172.16.0.2, static metric [1/0]

RTA(config)#undebug all

Static Routing

Configuring all of the static routes• Notice that the intermediate-address is always the next-hop ip address.

• This does not always have to be the case, and we will look at other options in the presentation on Static Routes- Additional Information

• Good idea to do a “copy running-config startup-config” if everything is working right.

• To verify the routes are in there, you can do a:

Router# show running-config

172.16.0.0/16 192.168.1.0/24

.1 .1.2 .2

RTA RTB RTC192.168.2.0/24

10.1.0.0/16

s0 s0 s1s1e0 e0

RTB(config)#ip route 192.168.2.0 255.255.255.0 172.16.0.1

RTB(config)#ip route 10.1.0.0 255.255.0.0 192.168.1.2

RTC(config)#ip route 192.168.2.0 255.255.255.0 192.168.1.1

RTC(config)#ip route 172.16.0.0 255.255.0.0 192.168.1.1

Static Routing

172.16.0.0/16 192.168.1.0/24

.1 .1.2 .2

RTA RTB RTC192.168.2.0/24

10.1.0.0/16

s0 s0 s1s1e0 e0

RTA#show ip route

S 10.1.0.0 [1/0] via 172.16.0.2

S 192.168.1.0/24 [1/0] via 172.16.0.2

RTA#ping 10.1.0.1

RTA#ping 192.168.1.2

RTA#ping 192.168.1.1

Static Routing

Recursive Lookup• The router knows it can get to 192.168.1.0/24 network by forwarding the packets to the

router at the ip address of 172.16.0.2• How does the router know how to get to the ip address 172.16.0.2?• It does a recursive lookup – first (1) by looking up the 192.168.1.0/24 network and

finding it needs to forward the packet to 172.16.0.2 – the router then (2) looks up the 172.16.0.0 network and sees it can forward it out the interface Serial 0.

RTA#show ip route

S 192.168.1.0/24 [1/0] via 172.16.0.2

172.16.0.0/16 192.168.1.0/24

.1 .1.2 .2

RTA RTB RTC192.168.2.0/24

10.1.0.0/16

s0 s0 s1s1e0 e0

Static Routing – Recursive Lookups

Recursive Lookup (continued)• We can take this even further.

• One route can be used to resolve the route of another.

• It doesn’t matter how the routes are resolved, whether they are directly connected, static or dynamic.

• Note: If an intermediate address cannot be resolved, that route and any routes it affects are not installed in the routing table.

RTA#show ip route

S 192.168.1.0/24 [1/0] via 172.16.0.2

S 10.1.0.0/16 [1/0] via 192.168.1.2

172.16.0.0/16 192.168.1.0/24

.1 .1.2 .2

RTA RTB RTC192.168.2.0/24

10.1.0.0/16

s0 s0 s1s1e0 e0

Note regarding recursive route lookups• Every route that does not reference an exit-interface must finally be resolved

via a route with an interface descriptor reference in the corresponding path descriptor – a route with an exit-interface.

• Static routes cannot be recursively resolved and will not be in the routing table.• Consider these three static routes:

Route1: ip route 10.1.0.0 255.255.0.0 20.1.1.1 Route2: ip route 20.1.0.0 255.255.0.0 30.1.1.1 Route3: ip route 30.1.0.0 255.255.0.0 10.1.1.1

• Route1 is resolved by Route2 which is resolved by Route3.• None of these routes are finally resolved via a route with an exit-interface.• This leads to endless recursion.• The routing table process will not permit these static routes to be entered in the

routing table.• Note: Static default routes (coming soon) can never be resolved via another

default route. (later)

Static Routes and the Routing Table Process

• Notice that the static route is entered into the routing table by the routing table process (debug ip routing) with a metric of 0.

172.16.0.0/16 192.168.1.0/24

.1 .1.2 .2

RTA RTB RTC192.168.2.0/24

10.1.0.0/16

s0 s0 s1s1e0 e0

RTA#debug ip routing

IP routing debugging is on

RTA#conf t

Enter configuration commands, one per line. End with CNTL/Z.

RTA(config)#undebug all

Static Routing – Routing Table Process

• Need only to use only an exit interface.

• For point-to-point serial interfaces, the next-hop address in the routing table is never used by the packet-delivery procedure, so it is not needed. (It could even reference a bogus IP address.)

• Notice that the static route appears in the routing table as directly connected, however it is still a static route with an administrative distance of 1.

172.16.0.0/16 192.168.1.0/24

.1 .1.2 .2

RTA RTB RTC192.168.2.0/24

10.1.0.0/16

s0 s0 s1s1e0 e0

RTA(config)#ip route 192.168.1.0 255.255.255.0 serial 0

RTA#show ip route

S 192.168.1.0/24 is directly connected, Serial0

Static Routing – Point-to-Point Links

Using an intermediate address instead of an exit-interface:

• If an intermediate address is used on a static route via a point-to-point link, it is only used to find the exit-interface,

• This recursive lookup is unnecessary and takes extra processing.

RTA#show ip route

S 192.168.1.0/24 [1/0] via 172.16.0.2

172.16.0.0/16 192.168.1.0/24

.1 .1.2 .2

RTA RTB RTC192.168.2.0/24

10.1.0.0/16

s0 s0 s1s1e0 e0

Static Routing – Point-to-Point Links

Using both an intermediate address instead and an exit-interface:

• Notice we changed 172.16.0.0 to an Ethernet link.

• Static routes via broadcast links, it is best to use both an exit interface and intermediate address.

• This saves the router from having to do a recursive route lookup for the intermediate address of 172.16.0.2, knowing the exit interface is Ethernet 0.

172.16.0.0/16 192.168.1.0/24

.1 .1.2 .2

RTA RTB RTC192.168.2.0/24

10.1.0.0/16

s0 s0 s1s1e0 e0

RTA(config)#ip route 192.168.1.0 255.255.255.0 eth 1 172.16.0.2

RTA#show ip route

S 192.168.1.0/24 [1/0] via 172.16.0.2 Ethernet1

Static Routing – Ethernet interfaces

Static Route Rule of Thumb

Static routes via point-to-point links

• It is best to configure static routes with only the exit interface.

• For point-to-point serial interfaces, the next-hop address in the routing table is never used by the packet-delivery procedure, so it is not needed. (It could even reference a bogus IP address.)

Static routes via broadcast networks such as Ethernet

• It is best to configure static routes with both the next-hop address and the exit-interface.

Using only an intermediate address

• “What about static routes referencing only intermediate network address? In short, try to avoid using them. The reason is that these static routes are not bound to any interface, rely on intermediate address resolvability, and thus converge more slowly. They can also create unexpected routing loops.” Alex Zinin, Cisco IP Routing

NOTE: Most of our examples in this course do not follow either of these rules-of-thumb – but you may want to use it on your network.

Static routes in the real-world• Soon we will learn about dynamic routing protocols (RIP, etc.), where routers can

learn automatically about networks, without the manual configuration of static routes.• Does this mean that static routes are never used in the real-world?• No! Static routes are used in conjunction with dynamic routing protocols.• It is common to use a static route where using a dynamic routing protocols would have

disadvantages or where it just not needed.

Common uses for Static Routes

Static routes in the real-world (continued)

• In the example above, there is only one route, link, between Cabrillo College’s network and the ISP.

• When there is only a single route to a network, this is known as a stub network.

• It is very common for the ISP to have a static route pointing to it’s customers’ networks, in this case Cabrillo College.

Cabrillo College

10.1.1.2/24

10.1.1.1/24

172.16.0.0/16

ip route 172.16.0.0 255.255.0.0 10.1.1.2

Static routes in the real-world (continued)• What about Cabrillo College and sending packets to the ISP – packets going to the Internet?• It is also common for customer networks to use a special kind of static route, known as a default

static route.• Of course we will examine this later throughout the rest of this course, but for now we specify the

network and mask as “0.0.0.0 0.0.0.0” (pronounced “quad-zero”).• This tells the router to forward all packets to this next-hop address (or exit interface) that do not have

an explicit route in the routing table.

Cabrillo College

10.1.1.2/24

10.1.1.1/24

172.16.0.0/16

ip route 172.16.0.0 255.255.0.0 10.1.1.2

ip route 0.0.0.0 0.0.0.0 10.1.1.1

Default

• Any packets not matching the routes 172.16.0.0/16 or 10.1.1.0/24 are sent to the router 10.1.1.1 – where it is now their “problem.”

Cabrillo College

10.1.1.2/24

10.1.1.1/24

172.16.0.0/16

ip route 172.16.0.0 255.255.0.0 10.1.1.2

ip route 0.0.0.0 0.0.0.0 10.1.1.1

RTB#show ip route

Gateway of last resort is 10.1.1.1 to network 0.0.0.0

C 10.1.1.0 is directly connected, Serial1

S* 0.0.0.0/0 [1/0] via 10.1.1.1

Default

Summarizing static routes

• There are many times when a single static route can replace several static routes.

• In other words, summarizing several static routes into a single static route.

Host 1

Host 2 Host 3

Hub Hub

172.16.2.0/24

172.16.3.0/24

172.16.1.0/24

192.168.1.0/24

192.168.2.0/24

SanJose2

SanJose1

Baypointe

.2.2172.16.0.0/24

Baypointe

• Let’s configure three static routes on Baypointe using either an intermediate-address or exit interface:

Baypointe(config)# ip route 172.16.1.0 255.255.255.0 192.168.1.2

Host 1

Host 2 Host 3

Hub Hub

172.16.2.0/24

172.16.3.0/24

172.16.1.0/24

192.168.1.0/24

192.168.2.0/24

SanJose2

SanJose1

Baypointe

.2.2172.16.0.0/24

Baypointe

• The three static routes can be summarized into a single route:

Baypointe(config)# ip route 172.16.1.0 255.255.255.0 192.168.1.2Baypointe(config)# ip route 172.16.2.0 255.255.255.0 192.168.1.2 Baypointe(config)# ip route 172.16.3.0 255.255.255.0 192.168.1.2

• Summarized route:

• The summarized route will now include all three subnets!

• Be sure to use the proper mask – 255.255.0.0!

• Using a 255.255.255.0 mask will only route for 172.16.0.0/24 subnet and not 172.16.1.0/24, 172.16.2.0/24 or 172.16.3.0/24.

Baypointe• Summarized route:

Advantages:

• Fewer routes in the routing table – faster routing table lookup.

• Subnets can be added and deleted on 172.16.0.0 network without having to change static route on Baypointe router.

Host 1

Host 2 Host 3

Hub Hub

172.16.2.0/24

172.16.3.0/24

172.16.1.0/24

192.168.1.0/24

192.168.2.0/24

SanJose2

SanJose1

Baypointe

.2.2172.16.0.0/24

Back to the curriculum…

Verify static routes

Copy running-config startup-config

Ping and Traceroute to troubleshoot

Dynamic Routing Protocols

Routed Protocols vs. Routing Protocols

Autonomous Systems

• An autonomous system (AS) is a collection of networks under a common administration sharing a common routing strategy.

• To the outside world, an AS is viewed as a single entity. The AS may be run by one or more operators while presenting a consistent view of routing to the external world.

• The American Registry of Internet Numbers (ARIN), a service provider, or an administrator assigns an identifying number to each AS.

Autonomous Systems

• Misinformation! This is not the same as an AS number used by ISPs! “Routing protocols, such as Cisco’s IGRP, require assignment of a unique, autonomous system number.”

Routing Protocols

• The goal of a routing protocol is to build and maintain the routing table.

• This table contains the learned networks and associated ports for those networks.

• Routers use routing protocols to manage information received from other routers, information learned from the configuration of its own interfaces, along with manually configured routes.

Types of Routing Protocols

• Distance Vector: RIP, IGRP, EIGRP

• Link State: OSPF, IS-IS

• Path Vector: BGP

• Note: IGRP and EIGRP are Cisco Proprietary

Distance Vector Routing Protocols

• “Routing by rumor”

• Each router receives a routing table from its directly connected neighbor routers.

Router B receives information from Router A.

Router B adds a distance vector number (such as a number of hops), which increases the distance vector.

Then Router B passes this new routing table to its other neighbor, Router C.

This same step-by-step process occurs in all directions between neighbor routers.

RTA RTB RTCNetwork W Network X Network Y Network Z

Routing Table (Distance) (Vector)Net. Hops Exit-int.W 0 <--X 0 -->

Routing Table (Distance) (Vector)Net. Hops Exit-int.X 0 <--Y 0 -->

Routing Table (Distance) (Vector)Net. Hops Exit-int.Y 0 <--Z 0 -->

Distance Vector Network Discovery

Routing UpdateNet. Hops Next-hop-addW 1 RTAX 1 RTA

Routing UpdateNet. Hops Next-hop-addX 1 RTBY 1 RTB

Routing UpdateNet. Hops Next-hop-addY 1 RTCZ 1 RTC

Routing Table (Distance) (Vector)Net. Hops Exit-int.W 0 <--X 0 -->Y 1 RTB

Routing Table (Distance) (Vector)Net. Hops Exit-int.X 0 <--Y 0 -->W 1 RTAZ 1 RTC

Routing Table (Distance) (Vector)Net. Hops Exit-int.Y 0 <--Z 0 -->X 1 RTB

Routing UpdateNet. Hops Next-hop-addW 1 RTAX 1 RTAY 2 RTA

Routing UpdateNet. Hops Next-hop-addX 1 RTBY 1 RTBW 2 RTBZ 2 RTB

Routing UpdateNet. Hops Next-hop-addY 1 RTCZ 1 RTCX 2 RTC

Routing UpdateNet. Hops Next-hop-addW 1 RTAX 1 RTAY 2 RTA

Routing UpdateNet. Hops Next-hop-addX 1 RTBY 1 RTBW 2 RTBZ 2 RTB

Routing UpdateNet. Hops Next-hop-addY 1 RTCZ 1 RTCX 2 RTC

Routing Table (Distance) (Vector)Net. Hops Exit-int.W 0 <--X 0 -->Y 1 RTBZ 2 RTB

Routing Table (Distance) (Vector)Net. Hops Exit-int.Y 0 <--Z 0 -->X 1 RTBW 2 RTB

ExistingRoutingTables

NewRoutingTables

Routing Table (Distance) (Vector)Net. Hops Exit-int.W 0 <--X 0 -->Y 1 RTBZ 2 RTB

Routing Table (Distance) (Vector)Net. Hops Exit-int.Y 0 <--Z 0 -->X 1 RTBW 2 RTB

RoutingTables

Convergence!

• Routing table updates occur when the topology changes. As with the network discovery process, topology change updates proceed step-by-step from router to router.

• With some routing protocols routing tables updates happen on a periodic basis.

Routing Protocol Metrics (costs_

• RIP – Hop Count

• IGRP and EIGRP – Bandwidth, Delay, Reliability, Load

• Cisco’s OSPF – Bandwidth

• IS-IS – Cost

• BGP – Number of AS or policy

Link State Routing Protocol Operations

• Link-state advertisements (LSAs) – A link-state advertisement (LSA) is a small packet of routing information that is sent between routers.

• Topological database – A topological database is a collection of information gathered from LSAs.

• SPF algorithm – The shortest path first (SPF) algorithm is a calculation performed on the database resulting in the SPF tree.

• Routing tables – A list of the known paths and interfaces.

• We will discuss this in more detail later when it will make much more sense.

Path Determination

A router determines the path of a packet from one data link to another, using two basic functions:

• A path determination function

• A switching function

Path Determination

• The switching function is the internal process used by a router to accept a packet on one interface and forward it to a second interface on the same router.

• A key responsibility of the switching function of the router is to encapsulate packets in the appropriate frame type for the next data link.

Configuring Dynamic Routing

GAD(config)#router rip

GAD(config-router)#network 172.16.0.0

More later!

Network command two things:• Tells the router which interfaces that will participate in this dynamic

routing protocol, which interfaces it will send and receive routing updates on.

• Tells other routers the networks in its routing updates that it is directly connect to.

• The network command is used on only directly connected networks.

• With RIP and IGRP, only need to use the classful address (no subnets).

Router(config)#router rip

Router(config-router)#network 172.16.0.0

More later!

Router(config)#router rip

Routing Protocols

• RIP – A distance vector interior routing protocol

• IGRP – Cisco's distance vector interior routing protocol

• OSPF – A link-state interior routing protocol

• EIGRP – Cisco’s advanced distance vector interior routing protocol

• BGP – A distance vector exterior routing protocol

Routing Protocols

Routing Information Protocol (RIP) was originally specified in RFC 1058. • It is a distance vector routing protocol. • Hop count is used as the metric for path selection. • If the hop count is greater than 15, the packet is discarded. • Routing updates are broadcast every 30 seconds, by default. Interior Gateway Routing Protocol (IGRP) is a proprietary protocol developed by

Cisco. • It is a distance vector routing protocol. • Bandwidth, load, delay and reliability are used to create a composite metric. • Routing updates are broadcast every 90 seconds, by default. EIGRP is a Cisco proprietary enhanced distance vector routing protocol. • It is an enhanced distance vector routing protocol. • Uses unequal-cost and equal-cost load balancing. • Uses a combination of distance vector and link-state features. • Uses Diffused Update Algorithm (DUAL) to calculate the shortest path. • NO! – [Routing updates are broadcast every 90 seconds or] as triggered by

topology changes.

Routing Protocols

Open Shortest Path First (OSPF) is a nonproprietary link-state routing protocol.

• It is a link-state routing protocol.

• Open standard routing protocol described in RFC 2328.

• Uses the SPF algorithm to calculate the lowest cost to a destination.

• Routing updates are flooded as topology changes occur.

Intermediate System to Intermediate System (IS-IS)

• IS-IS is an Open System Interconnection (OSI) routing protocol originally specified by International Organization for Standardization (ISO) 10589.

• It is a link-state routing protocol.

Border Gateway Protocol (BGP) is an exterior routing protocol.

• It is a distance vector (or path vector) exterior routing protocol

• Used between ISPs or ISPs and clients.

• Used to route Internet traffic between autonomous systems.

IGP vs EGP

• Much of this information is too early to discuss.

• Interior routing protocols are designed for use in a network whose parts are under the control of a single organization.

• An exterior routing protocol is designed for use between two different networks that are under the control of two different organizations.

Distance Vector Algorithm

• Distance vector algorithms (also known as Bellman-Ford algorithms) call for each router to send all or some portion of its routing table only to its neighbors.

• Distance vector algorithms perform routing decisions based upon information provided by neighboring routers.

• Distance vector protocols use fewer system resources but can suffer from slow convergence and may use metrics that do not scale well to larger systems.

Link State Algorithm

• Link-state algorithms (also known as shortest path first algorithms) flood routing information to all routers in the internetwork that creates a map of the entire network.

• Misinformation, this is not the reason why. – “Because they converge more quickly than distance vector protocols, link-state algorithms are less prone to routing loops.”

Summary

Ch. 6 – Routing and Routing Protocols

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