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Why RIP Doesn't Support Discontiguous NetworksA 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. Figure 2-7. An Example of 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:
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:
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 1137.99.0.0/24 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 |
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