Redistribution VLSM networks to BGP neighbour with summarized 24 bit mask network

Question

I received public BGP AS with network with 24 bit mask.

Is it possible, to advertise on BGP router VLSM networks summarized to 24-bit mask (not for class membership)?

Example: I receive 112.112.112.0/24 network with BGP AS (random network for example). I would like to divide this in my network to smaller subnets (for example with 28 bit mask), but my BGP peer doesn't accept advertised networks with higher subnet mask than 24 bit. As we know auto-summary summarize the advertised networks based on prefix for network class and in this example when we will use a auto-summary it redistribute a network 112.112.112.0 with 8 bit mask, so the result is 112.0.0.0/8 and it is wrong. How can I do it properly?

Answer 1

Yes, you can do this. Use the BGP aggregate-address command. It sounds like you want to use the summary-only option:

aggregate-address 112.112.112.0 255.255.255.0 summary-only

As long as even one of the networks comprising the summary are in the routing table, it will advertise the aggregate.

Cisco has documents that explain this. For example, Understanding Route Aggregation in BGP.