Problem: RIP-2 Routing Table Is Huge- Cause: Autosummarization Is Off

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Problem: RIP-2 Routing Table Is Huge ‚ Cause: Autosummarization Is Off

When a RIP update crosses a major network, it summarizes to the classful boundary. For example, 131.108.1.0, 131.108.2.0, and 131.108.3.0 will be autosummarized to 131.108.0.0/16 when advertised to a router with no 131.108.X.X addresses on its inter-faces. Disabling the autosummarization feature increases the size of the routing table. In some situations, this feature must be turned off (for example, if discontiguous networks exist, as discussed earlier).

Figure 3-39 shows the flowchart to follow to solve this problem based on this cause.

Figure 3-39. Flowchart to Resolve a Large RIP-2 Routing Table

Debugs and Verification

Example 3-103 shows the configuration on R2 that produces this problem. In this config-uration, R2 has autosummary turned off.

Example 3-103 Disabling Autosummarization Under RIP for R2
 R2#  router rip   version 2   network 132.108.0.0   network 131.108.0.0    no auto-summary   

Example 3-104 shows R1's routing table. This routing table has only four routes, but in a real network with the configuration in Example 3-103, there could be several hundred routes. R1 is receiving every subnet of 131.108.0.0/16. In this example, these are only three, but it can be much, much worse .

Example 3-104 Router R1 Routing Table Shows Subnetted Routes in the Routing Table
 R1#  show ip route rip  131.108.0.0/24 is subnetted, 3 subnets  R       131.108.3.0 [120/1] via 132.108.1.2, 00:00:24, Serial3   R       131.108.2.0 [120/1] via 132.108.1.2, 00:00:24, Serial3   R       131.108.1.0 [120/1] via 132.108.1.2, 00:00:24, Serial3  R1# 

Solution

Because the autosummarization feature is disabled under the RIP configuration of R2, R1 sees the subnetted routes in the routing table. When this feature is enabled, all the subnetted routes will go away.

Example 3-105 shows the altered configuration of R2. In this configuration, autosummarization is on, to reduce the size of the routing table. Because this is the default, you will not see it in the configuration. The command to enable autosummarization is auto-summary under router rip.

Example 3-105 R2 Uses Autosummarization to Reduce Routing Table Size
 R2#  router rip   version 2   network 132.108.0.0   network 131.108.0.0  

Example 3-106 shows the reduced size of the routing table.

Example 3-106 Autosummary Reduces the Routing Table Size for Router R1
 R1#  show ip route rip   R    131.108.0.0/16 [120/1] via 132.108.1.2, 00:00:01, Serial3  R1# 
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Troubleshooting IP Routing Protocols
Troubleshooting IP Routing Protocols (CCIE Professional Development Series)
ISBN: 1587050196
EAN: 2147483647
Year: 2002
Pages: 260

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