Cisco IOS Cookbook, 2nd Edition

Reducing the Size of the Received Routing Table

Problem

You want to summarize the incoming routing information to reduce the size of your routing table.

Solution

One of the easiest ways to reduce your routing table size is to filter out most of the external routes and replace them with a default. To do this, you first create a static default route pointing to some known remote network. If this remote network is up, then you can safely assume that your ISP is working properly. Then you simply filter out all of the remaining uninteresting routes:

Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#ip route 0.0.0.0 0.0.0.0 192.168.101.0 1
Router1(config)#ip route 0.0.0.0 0.0.0.0 192.168.102.0 2
Router1(config)#ip prefix-list CREATE-DEFAULT seq 10 permit 192.168.101.0/24
Router1(config)#ip prefix-list CREATE-DEFAULT seq 20 permit 192.168.102.0/24
Router1(config)#router bgp 65500
Router1(config-router)#neighbor 192.168.1.5 remote-as 65520
Router1(config-router)#neighbor 192.168.1.5 prefix-list CREATE-DEFAULT in
Router1(config-router)#exit
Router1(config)#end
Router1#

Discussion

For most typical Internet connections, you will need to drastically reduce the amount of routing information that you receive. A typical Internet backbone router needs to support BGP routes for well over 100,000 prefixes. So unless you are operating the ISP and need to support a large fraction of the public address space, it is a good idea to cut out as much as possible. ...