Impact of Auto Summary on Injected Routes by Network Command 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 network command..

If following is the routing table,

& if following is the BGP configuration,

Following will be the result;

With auto-summary command;

There will be nothing changed..

Let's add a network command like network 1.1.0.0

Nothing will happen in the BGP table..

Now let's remove that command and add the real classful summary with the command..

Ah now there is something changed to see..

You can see the classful NLRI is injected along with the specific NLRI, & even if you removed network 1.1.1.1 mask 255.255.255.255 the classful NLRI will be advertised.

What is really happening here is that the router is only looking at it's routing table, not the BGP configuration to match a subnet route. Because the IP routing table has 1 directly connected route to it's loopback interface for the subnet of 1.1.1.1/32, it is advertising the 1.0.0.0 in BGP. If we change the IP of the loopback to a something else, ex:- 2.2.2.2/32, the BGP table will be shown empty.