Why RIP Doesn't Support Discontiguous Networks

A discontiguous network is comprised of a major network separated by another major network. In Figure 2-7, network 131.108.0.0 is separated by a subnet of network 137.99.0.0; here, 131.108.0.0 is a discontiguous network.

RIP is a classful protocol. Whenever RIP advertises a network across a different major network boundary, RIP summarizes the advertised network at the major network boundary. In Figure 2-7, when Router 1 sends an update containing 131.108.5.0 to Router 2 across 137.99.88.0, it converts 131.108.5.0/24 into 131.108.0.0/16. This process is called autosummarization.

Router 1 takes the following steps before sending an update to Router 2:

1. Is 131.108.5.0/24 part of the same major network as 137.99.88.0/24, which is the subnet assigned to the interface that's sourcing the update?

2. No. Router 1 summarizes 131.108.5.0/24 and advertises the route 131.108.0.0/16.

The debug ip rip command output on Router 1 shows the update sent by Router 1, as demonstrated in Example 2-4.

Example 2-4 debug ip rip Command Output Reveals RIP Update Information Sent by Router 1 in Figure 2-7

Router1#

debug ip rip

RIP: sending v1 update to 255.255.255.255 via Serial0 (137.99.88.2)

     network 131.108.0.0, metric 1

Router 2 goes through the following steps before accepting the update from Router 1:

1. Is the major network received (131.108.0.0) the same as the major network of 137.99.88.0/24, which is the subnet assigned to the interface that received the update?

2. No. Do any subnets of this major network already exist in the routing table known from interfaces other than that which received the update?

3. Yes. Router 2 ignores the update.

Again, debug ip rip command output on Router 2 shows the update received by Router 2, as demonstrated in Example 2-5.

Example 2-5 debug ip rip Command Output Reveals RIP Update Information Received by Router 2 in Figure 2-7

Router2#

debug ip rip

RIP: received v1 update from 137.99.88.1 on Serial0

     131.108.0.0 in 1 hops

The routing table of Router 2, as demonstrated in the show ip route command output in Example 2-6, shows that the update was ignored. The only entry for any subnetwork or network on 131.108.0.0 is the one directly connected to Ethernet0.

Example 2-6 show ip route Command Output Reveals That the Routing Table for Router 2 in Figure 2-7 Does Not Reflect the Advertised Route Sent by Router 1

137.99.0.0/24 is subnetted, 1 subnets

    C       137.99.88.0 is directly connected, Serial0

         131.108.0.0/24 is subnetted, 3 subnets

    C       131.108.2.0 is directly connected, Ethernet0

To avoid having updates ignored, configure a static route on both routers that points toward the specific subnets. For example, on Router 1, configure the following:

Router1(config)#

ip route 131.108.2.0 255.255.255.0 137.99.88.1

On Router 2, configure the following:

Router2(config)#

ip route 131.108.5.0 255.255.255.0 137.99.88.2