bgp inject-map

[Kashish_Patel]

I am confused about the use case of bgp inject-maps. If the goal is to inject specific prefixes, then why not just add an ip route <network> <next-hop> statement for more specific prefix (so that the specific prefix appears in routing table) and then add a network statement under router bgp so that the specific prefix appears in bgp table also? Could any expert help?

I was reading about the feature at  https://blog.initialdraft.com/archives/3527/

[Jon Marshall · Accepted solution]

Kashish

You could indeed just add more speciifc routes to the IP routing table and then configure "network ..." commands under BGP. But then because the route is always in the IP routing table it would always be advertising the more specific routes.

The BGP inject-maps feature is also known as BGP conditional advertising ie. you may only want to advertise the more specific prefixes if you are actually receiving the less specific prefix from a neighbor. Because if you stop receiving that less specific prefix then you do not want traffic coming to your router as it will be dropped.

So with BGP inject-maps you can say only advertise the more specific prefixes as long as the router is receiving the less specific prefix. If the less specific prefix is no longer received then do not advertise the more specific prefixes.

You couldnt do this if you simply added routes to the routing table.

Note i referred to less specific rather than summary route as it doesn't actually have to be a summary route ie. it could be a class C for example but you are advertiing out subnets within the class C.

Jon

[Kashish_Patel]

Thanks Jon. That makes sense now.