Problem: RIP-2 Routing Table Is Huge- Cause: ip summary-address Is Not Used

Problem: RIP-2 Routing Table Is Huge ‚ Cause: ip summary-address Is Not Used

Figure 3-40 shows the network setup that could produce a large routing table.

Figure 3-40. Network Setup That Could Generate a Large Routing Table

Figure 3-40 shows that 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 autosummar-ization cannot play any role to solve the problem of receiving a subnet route that could be summarized. The autosummarization feature could have worked only if the R1, R2 link was in a different major network.

Figure 3-41 shows the flowchart to follow to solve this problem based on this cause.

Figure 3-41. Flowchart to Resolve a Large RIP-2 Routing Table

Debugs and Verification

Example 3-107 shows that in the configuration of R2, the ip summary-address command is not used under the Serial 1 interface to summarize the routes.

Example 3-107 R2's Serial Interface Is Not Configured to Summarize Routes

 R2#  interface Serial1   ip address 131.108.4.2 255.255.255.0   !   router rip   version 2   network 131.108.0.0  

Example 3-108 shows the routing table of R1. In this example, there are only three routes. In a real network, however, the number could be worse based on the configuration in Example 3-107.

Example 3-108 R1 Routing Table Shows Subnetted Routes

 R1#  show ip route rip  131.108.0.0/24 is subnetted, 3 subnets R       131.108.3.0 [120/1] via 131.108.4.2, 00:00:04, Serial3 R       131.108.2.0 [120/1] via 131.108.4.2, 00:00:04, Serial3 R       131.108.1.0 [120/1] via 131.108.4.2, 00:00:04, Serial3 R1# 

Solution

In the situation described in the preceding section, autosummary is on but is not helpful because the whole network is 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.

Example 3-109 shows the new configuration that solves this problem. This configuration reduces the size of the routing table. This command can be used with different masks so that, if a network has contiguous blocks of a subnet, the router could be configured to summarize subnets into smaller blocks. This then would reduce the routes advertised to the RIP network.

Example 3-109 Manual Summarization with RIP

 R2#:  interface Serial1   ip address 131.108.4.2 255.255.255.0    ip summary-address rip 131.108.0.0 255.255.252.0    !   router rip   version 2   network 131.108.0.0  

Based on the preceding configuration, R2 will summarize the RIP route on the Serial 1 interface. Any network subnet that falls in the 131.108.0.0 network will be summarized to one 131.108.0.0 major network, and its mask will be 255.255.252.0. This means that R2 will announce only a single summarize route of 131.108.0.0/22 and will suppress the subnets of 131.108.0.0.

Example 3-110 shows the routing table of Router R1 with a reduced number of entries as a result of summarization.

Example 3-110 R1's Routing Table Is Reduced as a Result of Summarization

 R1#  show ip route rip   R    131.108.0.0/22 [120/1] via 131.108.4.2, 00:00:01, Serial3  R1#