Impact of Auto Summary on Injected Routes by Redistribution in BGP

Auto-summary is disabled by default in 12.3 and later Cisco IOS versions. Let's see the behavior of the router when it is enabled. Looks like it is breaking some general rules of BGP route injection.

Unlike IGPs, the BGP auto-summary router sub command causes BGP to summarize only those routes injected because of redistribution on that router. When it is configured, it does not look for the classful network boundaries in the topology nor routes already in the BGP table (as it does normally), it simply looks for the routes injected into BGP because of the redistribute and the network command on the same router.

Redistribute command: If any subnets of a classful network would be redistributed, do not redistribute subnets, but instead redistribute a route for the classful network..

Network command: If a network command lists a classful network, and there is atleast one route in IP routing table to a subnet of it, inject a route to the classful network..

In this post, let's experience the behavior with redistribution..

If following is the routing table,

& if following is the BGP configuration,

Following routes will be injected via the redistribute connected command..

When auto-summary is configured,

You can see, 1.1.1.1/32 is removed and 1.0.0.0 is placed..