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Chapter 5

RIP version 1

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RIPv1: A Distance Vector,

Classful Routing ProtocolBackground and Perspective

RIPv1 Characteristics and Message Format

RIP Operation

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3

RIPv1: Distance Vector, Classful Routing Protocol

The first protocol used was Routing Information Protocol (RIP).

RIP still popular: simple. Why learn RIP?

Still in use today.

Help understand fundamental concepts and comparisons of

protocols

such as classful (RIPv1) and classless (RIPv2)

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4

Background and Perspective

RIP is not a protocol on the way out.

In fact, an IPv6 form of RIP called RIPng (next generation) is now

available..

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5


Note:

The first version of RIP is often called RIPv1 to distinguish it from

RIP version 2 (RIPv2).

However, both versions share many of the same features. When

discussing features common to both versions, we will refer to

RIP. When discussing features unique to each version, we will

use RIPv1 and RIPv2.

RIPv2 is discussed in Chapter 7.

RIP characteristics:

Distance vector routing protocol.

Metric: hop count Advertised routes with hop counts greater than 15 are

considered unreachable.

Response messages (routing table updates) are broadcast every

30 seconds. (RIPv2 uses multicasts)

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Next slide

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RIP Message Format: Route Entry

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RIP Operation

Startup

1. Each RIP-configured interface sends out a Request message

Asking for their complete routing tables.

2. A Response messageis sent back by RIP-enabled neighbors.

If new route: Installs in routing table.

If existing route: Replace if better hop count.

Startup router then sends a triggered updateout all RIP-enabled interfaces

containing its own routing table so that RIP neighbors can be informed ofany new routes.

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IP Address Classes and Classful Routing

RIPv1:

Classful routing protocol.

Does notsend subnet mask in update.

A router either uses the subnet mask: (discussed later)

configured on a local interface or

applies the default classful subnet mask

Because of this limitation, RIPv1 networks cannot be discontiguous, nor can

they implement VLSM.

No subnet

mask

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Administrative Distance

RIP has a default administrative distance of 120.

When compared to other interior gateway protocols, RIP is the least-

preferred routing protocol.

Note: This is irrelevant because you usually do not run multiple routing

protocols in the same domain, and even if you did you can modify these

AD values.

R3# show ip route

R 192.168.1.0/24 [120/1] via 192.168.6.2, 00:00:05, Serial0/0/0

R3# show ip protocols

Routing Protocol is rip

Routing Information Sources:

Gateway Distance Last Update

192.168.6.2 120 00:00:10

Distance: (default is 120)

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Basic RIPv1 Configuration

RIPv1 Scenario A

Enable RIP: router rip Command

Specifying Networks

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Notice that this topology uses five Class C network addresses.

Remember, RIPv1 is a classful routing protocol

We will see that the class of the network is used by RIPv1 to determine the

subnet mask.

RIPv1 Scenario A

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Enabling RIP: router rip Command

Enter router configuration mode for RIP, enter router ripat the

global configuration prompt. Notice that the prompt changes.

R1# conf t

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

R1(config)# router ?

bgp Border Gateway Protocol (BGP)

egp Exterior Gateway Protocol (EGP)

eigrp Enhanced Interior Gateway Routing Protocol (EIGRP)

igrp Interior Gateway Routing Protocol (IGRP)isis ISO IS-IS

iso-igrp IGRP for OSI networks

mobile Mobile routes

odr On Demand stub Routes

ospf Open Shortest Path First (OSPF)

rip Routing Information Protocol (RIP)

R1(config)# router rip

R1(config-router)#

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Enabling RIP: router rip Command

router rip

Does not directly start the RIP process.

Provides access to configure routing protocol settings.

No routing updates are sent until additional commands are

configured.

no router rip

To remove the RIP routing process from a device

Stops the RIP process

Erases all existing RIP configuration commands.

R1# conf t


R1(config-router)#

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Specifying Networks

To enable RIP routing for a network, use the networkcommand in

router configuration mode

Enter the classful network address for each directly connected

network.

Router(config-router)# networkdirectly-connected-classful-

network-address

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Specifying Networks

The network command performs the following functions:

Enables RIP on all interfaces that belong to a specific network.

Associated interfaces will now both send and receive RIP

updates.

Advertises the specified network in RIP routing updates sent to

other routers every 30 seconds.

Router(config-router)# network

directly-connected-classful-network-address

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Specifying Networks

If you enter a subnet or host IP address, IOS automatically converts

it to a classful network address.

For example, if you enter the command network 192.168.1.32,

the router will convert it to network 192.168.1.0.

R1(config)#router rip

R1(config-router)#network 192.168.1.0









Only directly connected classful network

addresses!

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Only directly connected classful network

addresses!

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Verification and

TroubleshootingVerifying RIP: show ip route

Verifying RIP: show ip protocols

Verifying RIP: debu ip rip

Passive Interfaces

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Verifying RIP: show ip route Command

R1# show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile,

Gateway of last resort is not set

R 192.168.4.0/24 [120/1] via 192.168.2.2, 00:00:02, Serial0/0/0

R 192.168.5.0/24 [120/2] via 192.168.2.2, 00:00:02, Serial0/0/0

C 192.168.1.0/24 is directly connected, FastEthernet0/0

C 192.168.2.0/24 is directly connected, Serial0/0/0

R 192.168.3.0/24 [120/1] via 192.168.2.2, 00:00:02, Serial0/0/0

An Rin the output indicates RIP routes.

Because this command displays the entire routing table, including

directly connected and static routes, it is normally the first command

used to check for convergence.

Routes might not immediately appear when you execute the

command because networks take some time to converge..

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R2# show ip route




R 192.168.5.0/24 [120/1] via 192.168.4.1, 00:00:12, Serial0/0/1

R 192.168.1.0/24 [120/1] via 192.168.2.1, 00:00:24, Serial0/0/0



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R3# show ip route





R 192.168.1.0/24 [120/2] via 192.168.4.2, 00:00:08, Serial0/0/1

R 192.168.2.0/24 [120/1] via 192.168.4.2, 00:00:08, Serial0/0/1

R 192.168.3.0/24 [120/1] via 192.168.4.2, 00:00:08, Serial0/0/1

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R1# show ip route

R 192.168.5.0/24 [120/2] via 192.168.2.2, 00:00:23, Serial0/0/0

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Verifying RIP: show ip protocols Command

Verifies that RIP routing is configured and running on

Router R2 At least one active interface with an associated network

command is needed before RIP routing will start.

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These are the timers that show when the next

round of updates will be sent out from thisrouter23 seconds from now, in the example.

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This information relates to filtering updates and

redistributing routes, if configured on this router. Filtering and redistribution are both CCNP-level

topics.

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Information about which RIP version is

currently configured and which interfaces areparticipating in RIP updates.

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Router R2 is currently summarizing at the classful networkboundary

By default, will use up to four equal-cost routes to load-balance.

Automatic summarization is discussed later in thischapter.

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Classful networks configured with the network

command are listed next. These are the networks that R2 will include in its

RIP updates. (with other learned routes)

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RIP neighbors

Gateway: Next-hop IP address of the neighbor that is sending R2updates.

Distance is the AD that R2 uses for updates sent by this neighbor.

Last Update is the seconds since the last update was receivedfrom this neighbor.

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Verifying RIP: debug ip rip Command

The debug command is a useful tool to help diagnose and resolve

networking problems, providing real-time, continuous information.

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Update coming in from R1 on interface Serial 0/0/0.

R1 only sends one route: 192.168.1.0.

No other routes are sent because doing so would violate the split horizonrule.

R1 is not allowed to advertise networks back to R2 that R2 previouslysent to R1.

RIP: receivedv1 update from 192.168.2.1 on Serial0/0/0

192.168.1.0 in 1 hops

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The next update that is received is from R3.

Because of the split horizon rule, R3 only sends one route: the 192.168.5.0

network.

RIP: receivedv1 update from 192.168.4.1 on Serial0/0/1

192.168.5.0 in 1 hops

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R2 sends out its own updates.

FastEthernet 0/0 interface:

Includes the entire routing table except for network 192.168.3.0,which is attached to FastEthernet 0/0.

RIP: sendingv1 update to 255.255.255.255 via FastEthernet0/0

(192.168.3.1)

RIP: build update entries

network 192.168.1.0 metric 2




Directly

Connected

Learned via

RIP from R1Learned via

RIP from R3

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R2 sends update to R3.

Three routes are included.

R2 does not advertise the network R2 and R3 share, nor does itadvertise the 192.168.5.0 network because of split horizon.

RIP: sendingv1 update to 255.255.255.255 via Serial0/0/1

(192.168.4.2)





Learned via

RIP from R1

DirectlyConnected

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R2 sends update to R1.

Three routes are included.

R2 does not advertise the network that R2 and R1 share, nor does itadvertise the 192.168.1.0 network because of split horizon.

In another 30 seconds, all the debug output will repeat (every 30 seconds).

RIP: sendingv1 update to 255.255.255.255 via Serial0/0/0

(192.168.2.2)





Directly

Connected

Learned via

RIP from R3

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To stop monitoring no debug ip ripor undebug all

But do you see a way to optimize RIP routing on R2?

Does R2 need to send updates out FastEthernet 0/0?

You will see in the next topic how to prevent unnecessary updates.

R2# undebug all

All possible debugging has been turned off

Got router?

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Some routers can have interfaces that do not connect to another

router.

You can use thepassive-interfacecommand with RIP toconfigure an interface not to send those updates.

Bandwidth is wasted transporting unnecessary updates.

All devices on the LAN must process the RIPv1 update up to the

transport layer.

Security risk (Authentication would is a better solution - later)

Got router?

Passive Interfaces

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Passive Interfaces

What about using on R2:

R2(Config-router)# no network 192.168.3.0

But then R2 would not advertise this LAN as a route in updates sent

to R1 and R3.

Correct solution is to use thepassive-interfacecommand

Router(config-router)#passive-interface interface-type interface-

number

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Passive Interfaces


R2(config-router)#passive-interface FastEthernet 0/0

X

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Passive InterfacesR2# show ip protocols

Interface Send Recv Triggered RIP Key-chain

Serial0/0/0 1 1 2

Serial0/0/1 1 1 2

Automatic network summarization is in effect

Routing for Networks:

192.168.2.0192.168.3.0

192.168.4.0

Passive Interface(s):

FastEthernet0/0

Routing Information Sources:

Gateway Distance Last Update

192.168.2.1 120 00:00:27

192.168.4.1 120 00:00:23

Distance: (default is 120)

FastEthernet 0/0 no longer

included

LAN network still included inRIP updates that are sent

FastEthernet 0/0 is a passive interface

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Automatic Summarization

Modified Topology B

Boundary Routers and Automatic Summarization

Processing RIP Updates

Sending RIP UpdatesAdvantages and Disadvantages of Automatic

Summarization

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Modified Topology: Scenario B

Fewer routes in a routing table means that the routing table process

can more quickly locate the route needed to forward the packet.

Summarizing several routes into a single route is known as route

summar izat ionor route aggregat ion.

Some routing protocols, such as RIP, automatically summarize

routes on certain routers.

172.30.0.0/16

192.168.4.0/24

192.168.5.0/24

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Modified Topology: Scenario B

Three classful

networks are used:

172.30.0.0/16

192.168.4.0/24

192.168.5.0/24

172.30.0.0/16

192.168.4.0/24

192.168.5.0/24

The 172.30.0.0/16network is subnetted intothree subnets:

172.30.1.0/24

172.30.2.0/24

172.30.3.0/24

The 192.168.4.0/24

network is

subnetted as a

single subnet

192.168.4.8/30.

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Configuration Changes for R1

R1(config)# interface fa0/0

R1(config-if)# ip address 172.30.1.1 255.255.255.0R1(config-if)# interface S0/0/0

R1(config-if)# ip address 172.30.2.1 255.255.255.0

R1(config-if)# no router rip


R1(config-router)# network 172.30.1.0



R1(config-router)# end

R1# show run

!router rip

passive-interface FastEthernet0/0

network 172.30.0.0

!

IOS automatically corrects

subnet entries to classful

network address

The no shutdownand

clock ratecommands are

not needed because these

commands are still

configured from Scenario A.

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Configuration Changes for R2R2(config)# interface S0/0/0


R2(config-if)# interface fa0/0


R2(config-if)# interface S0/0/1



R2(config)# router ripR2(config-router)# network 172.30.0.0




R2# show run

!

router rip


network 172.30.0.0

network 192.168.4.0!

IOS automatically correctssubnet entries to classful

network address

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Configuration Changes for R3

R3(config)# interface fa0/0

R3(config-if)# ip address 192.168.5.1 255.255.255.0R3(config-if)# interface S0/0/1








R3# show run

!router rip


network 192.168.4.0

network 192.168.5.0

!

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RIPis a classful routing protocol that automatical ly summarizes

classfu l networks across major network bou ndar ies.

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R2 has interfaces in more than one major classful network.

This makes R2 a boundary router in RIP.

Both Serial 0/0/0 and FastEthernet 0/0 interfaces on R2 are inside the172.30.0.0 boundary.

The Serial 0/0/1 interface is inside the 192.168.4.0 boundary.

Boundary routers summarize RIP subnets from one major network to theother, updates for the 172.30.1.0, 172.30.2.0, and 172.30.3.0networks willautomatically be summarized into 172.30.0.0when sent out R2s Serial

0/0/1 interface.

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Processing RIP Updates

Classful routing protocolssuch as RIPv1 do not include the subnet

maskin the routing update.

However, the routing table includes RIPv1 routes with both the network

address and the subnet mask.

So how does a router running RIPv1 determine what subnet mask i t

shou ld apply to a route when adding i t to th e rout ing table?

R2# show ip route

172.30.0.0/24is subnetted, 3 subnets

R 172.30.1.0 [120/1] via 172.30.2.1, 00:00:18, Serial0/0/0

C 172.30.2.0 is directly connected, Serial0/0/0C 172.30.3.0 is directly connected, FastEthernet0/0


C 192.168.4.8 is directly connected, Serial0/0/1

R 192.168.5.0/24[120/1] via 192.168.4.10, 00:00:16, Serial0/0/1

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Rules for Processing RIPv1 Updates

The following two rules govern RIPv1 updates:

If a routing updateand the interfaceon which it is received

belong to the same major network, the subnet mask of theinterfaceis applied to the network in the routing update.

If a routing updateand the interfaceon which it is received

belong to different major networks, the classful subnet mask

of the networkis applied to the network in the routing update..

Routing Updateand Interface Routing Update Subnet Mask

Same Classful Major Network Use mask of interface

Different Classful Major Network Use default classful mask

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Example of RIPv1 Processing Updates

Same classful network as theincoming update.

Update: 172.30.1.0in 1 hops

Interface received:

Serial 0/0/0 - 172.30.2.2/24

Same classful network address(172.30.0.)

Applies subnet mask of its S0/0/0interface, /24.

The 172.30.1.0 /24 subnet wasadded to the routing table.

R2# debug ip rip (selected output)

RIP: received v1 update from 172.30.2.1 on Serial0/0/0

172.30.1.0in 1 hops

R2# show ip route (selected output)


R 172.30.1.0[120/1] via 172.30.2.1, 00:00:18, Serial0/0/0

172.30.1.0

172.30.2.2/24

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Sending RIP UpdatesR2# debug ip rip

RIP protocol debugging is on

RIP: sending v1 update to 255.255.255.255 via Serial0/0/0(172.30.2.2)RIP: build update entries




RIP: sending v1 update to 255.255.255.255 via Serial0/0/1 (192.168.4.9)



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Sending RIP Updates

R2# debug ip rip


RIP: sending v1 update to 255.255.255.255 via Serial0/0/0(172.30.2.2)








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Sending RIP Updates

R2# debug ip rip


RIP: sending v1 update to 255.255.255.255 via Serial0/0/0(172.30.2.2)








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Sending RIP Updates

172.30.3.0

192.168.4.0

192.168.5.0

172.30.0.0

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Determining the mask and network address

Receiving an Update: Determining subnet mask for routing table

What is the major classful network address of the receiving interface? What is the major classful network address of the network in the routing

update?

Are they the same major classful network address?

Yes: Apply subnet mask of the receiving interface for this network

address in the routing table. No: Apply classful subnet mask for this network address in the

routing table.

Sending an Update: Determining whether or not to summarize route sent

What is the major classful network address of the sending interface?

What is the major classful network address of the network in the routingupdate?

Are they the same major classful network address?

Yes: Send subnet network address

No: Send summary addressthe classful network address

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Verifying Routing Updates

R1# show ip route



C 172.30.1.0 is directly connected, FastEthernet0/0


R 172.30.3.0[120/1] via 172.30.2.2, 00:00:17, Serial0/0/0

R 192.168.4.0/24[120/1] via 172.30.2.2, 00:00:17, Serial0/0/0R 192.168.5.0/24[120/2] via 172.30.2.2, 00:00:17, Serial0/0/0

R3# show ip route


R 172.30.0.0/16[120/1] via 192.168.4.9, 00:00:15, Serial0/0/1

192.168.4.0/30 is subnetted, 1 subnets



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Classful routing protocols do not support VLSM

Routers running RIPv1 are l imi ted to us ing the same su bnet mask for all

subn ets wi th the same classfu l network.

As you will learn in later chapters, classless routing protocols such as RIPv2

allow the same major (classful) network to use different subnet masks on

different subnets, better known as variable-length subnet masking (VLSM)..

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Advantages of Automatic Summarization

Smaller routing updates are sent and received, which uses less bandwidth

for routing updates between R2 and R3.

R3 has a single route for the 172.30.0.0/16 network, regardless of howmany subnets there are or how it is subnetted.

Using a single route results in a faster lookup process in the routing

table for R3.

R3# show ip route


R 172.30.0.0/16[120/1] via 192.168.4.9, 00:00:15, Serial0/0/1




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Disadvantage of Automatic Summarization

Discon t iguous network, two or more subnets separated by at least

one other major network.

172.30.0.0/16 is a discontiguous network.

172.30.0.0/16 172.30.0.0/16

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Discontiguous Networks Do Not Converge with RIPv1


R1(config-router)# network 172.30.0.0R1(config-router)# network 209.165.200.0







RIPv1 configuration is correct, but it is unable to determine all the

networks in this discontiguous topology.

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Both routers, however, will advertise the 172.30.0.0 major network

address, a summary route to R2.

172.30.0.0/16 172.30.0.0/16

172.30.0.0 172.30.0.0

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Discontiguous Networks Do Not Converge with RIPv1R2# show ip route

R 172.30.0.0/16 [120/1] via 209.165.200.234, 00:00:14, Serial0/0/1

[120/1] via 209.165.200.229, 00:00:19, Serial0/0/0

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R2 has two equal-cost paths to the 172.30.0.0 network.

R2 will load-balance traffic destined for any subnet of 172.30.0.0.

This means that R1 will get half of the traffic and R3 will get the other half of the

traffic, whether or not the destination of the traffic is for one of their LANs.

R2# show ip route

R 172.30.0.0/16 [120/1] via 209.165.200.234, 00:00:14, Serial0/0/1

[120/1] via 209.165.200.229, 00:00:19, Serial0/0/0

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Classful routing protocols do not support discontiguous networks becausethey do not include the subnet mask in the routing update.

Classless routing protocols (RIPv2, EIGRP, OSPF, IS-IS, BGP) do support

discontiguous networks.

R2# show ip route

R 172.30.0.0/16 [120/1] via 209.165.200.234, 00:00:14, Serial0/0/1

[120/1] via 209.165.200.229, 00:00:19, Serial0/0/0

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Default Route and RIPv1

Modified Topology C

Propagating the Default Route in RIPv1

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Modified Topology: Scenario C

Default routes are used by routers to represent all routes that are notspecifically in the routing table.

A default route is commonly used to represent routes that are not in the

locally administered network, such as the Internet..

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Default Routes

In todays networks, customers do not necessarily have to exchange routing

updates with their ISP.

Customer routers that connect to an ISP do not need a listing for everyroute on the Internet.

Instead, these routers have a default routethat sends all traffic to the ISP

router when the customer router does not have a route to a destination.

The ISP configures a static route pointing to the customer router for

addresses inside the customers network.

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Configuration Changes for R2 and R3


R2(config-router)# no network 192.168.4.0

R2(config-router)# exit

R2(config)# ip route 0.0.0.0 0.0.0.0 serial 0/0/1

R3(config)# no router rip

R3(config)# ip route 172.30.0.0 255.255.252.0 serial 0/0/1

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Routing Table

R1 has all 172.30.0.0/24 subnets, but will drop packets for all other

networks.

No default route (coming)

R1# show ip route





R 172.30.3.0 [120/1] via 172.30.2.2, 00:00:05, Serial0/0/0

R ti T bl

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Routing Table

R2 has routes for 172.30.0.0/16 subnets.

R2 has static default route for all other networks

R2# show ip route

Gateway of last resort is0.0.0.0 to network 0.0.0.0


R 172.30.1.0 [120/1] via 172.30.2.1, 00:00:03, Serial0/0/0



192.168.4.0/30 is subnetted, 1 subnetsC 192.168.4.8 is directly connected, Serial0/0/1

S* 0.0.0.0/0 is directly connected, Serial0/0/1

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Routing Table

R3 has static route for 172.30.0.0/16 network.

Doesnt matter if or how 172.30.0.0/16 is subnetted, R3 will forward packets

to R2.

R3# show ip route



S 172.30.0.0 is directly connected, Serial0/0/1




P ti th D f lt R t i RIP 1

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Can configure static default route on every router but:

inefficient

does not react to topology changes In many routing protocols, including RIP, you can use the default-

information originate command in router configuration mode to specify

that th is rou ter is to or igin ate default info rmat ion , by propagat ing the

stat ic default rou te in RIP updates.

R1# show ip route





R 172.30.3.0 [120/1] via 172.30.2.2, 00:00:05, Serial0/0/0


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R2(config-router)# default-information originateR2(config-router)# end

R2# debug ip rip

RIP: sending v1 update to 255.255.255.255 via Serial0/0/0

(172.30.2.2)


subnet 0.0.0.0 metric 1

subnet 172.30.3.0 metric 1


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The static default route on R2 has been propagated to R1 in a RIP update.

R1 has connectivity to the LAN on R3 and any destination on the Internet.

R1# show ip route

* - candidate default, U - per-user static route, o - ODR

Gateway of last resort is 172.30.2.2 to network 0.0.0.0



R 172.30.3.0 [120/1] via 172.30.2.2, 00:00:16, Serial0/0/0


R* 0.0.0.0/0 [120/1] via 172.30.2.2, 00:00:16, Serial0/0/0

ccnaexp5

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