Content

| 5.4 |  | Troubleshooting RIP |
| 5.4.7 |  | Large routing tables |

Large routing tables can be problematic for a network by increasing the amount of bandwidth required for routing updates as well as leading to increased memory requirements. Figure shows an example of a network that could produce a large routing table. In this example, there are only three routes. In a real network, however, the number of networks could be much higher and the problem of a large routing table, much worse.

R2 is announcing several subnets of 131.108.0.0 network. Notice that the link between R1 and R2 is also part of the 131.108.0.0 network, so auto-summarization cannot play any role to solve the problem of receiving a subnet route that could be summarized. The auto-summarization feature can only work if the link between R1 and R2 is on a different major network.

Debugs and Verification
Figure shows that in the configuration of R2, the ip summary-address command is not used under the Serial 1 interface to summarize the routes.

Figure shows the routing table of R1.

Solution
In this situation, autosummary is on but is not helpful because the whole network in within one major network. Imagine a network with Class B address space with thousands of subnets. Autosummary cannot play any role here because no major network boundary is crossed. A new feature of summarization was introduced in RIP starting with Cisco IOS Software Release 12.0.7T. This feature is similar to EIGRP manual summarization.

Figure shows the new configuration that solves this problem using the ip summary-address rip command. This configuration reduces the size of the routing table. This command can be used with different masks, so if a network has contiguous blocks of a subnet, the router could be configured to summarize subnets into smaller blocks. This would reduce the routes advertised to the RIP network.

Based on the preceding configuration, router R2 will summarize the RIP routes on the Serial 1 interface. Any network subnet that falls in the 131.108.0.0 to 131.108.3.0 range of networks will be summarized as a single 131.108.0.0/22 major network. This means that R2 will announce only a single summarized route as 131.108.0.0/22 and suppress the 131.108.1.0/24, 131.108.2.0/24, and 131.108.3.0/24 subnets.

Figure shows the routing table of router R1 with a reduced number of entries as a result of the summarization.