BGP address aggregation

[frank.meurice]

Hello,

I was ready the document on bgp address aggregation (summarisation) to find why some routes are still advertised while there is a aggregate command that should cover them. Apparently it was beceause the "summary-only" was missing.

Syntax: aggregate-address address mask [as-set] [summary-only]

Now can anybody shed a light on the difference between the impact of the 2 versions of the command?

aggregate-address a.a.a.a m.m.m.m

and

aggregate-address a.a.a.a m.m.m.m summary-only

Thank you for your assistance.

[cadet alain]

Hi,

I think you answered yourself to this question. By default BGP sends the summary and the component routes, if you want to advertise only the summary then use summary-only. On the aggregating router you'll see the component subnets marked as suppressed( small s at the left far side).

Regards.

Alain

Don't forget to rate helpful posts.

[frank.meurice]

Alain,

thank you for the reply.

I guess I didn't really posted my question correctly.

Why would you want to have both the summary and component routes in your database?

Or you have all the details,Or you have a summary which is easier on the routers and bandwith.

Don't see the scenario where having both is usefull.

Thx for the update ...

[milan.kulik]

Hi Frank,

imagine following sceanrio:

You are peering to several BGP neighbors.

You are receiving 10000 of 10.x.x.x/y prefixes from them.

And you need to advertise only the summary  prefix 10.0.0.0/8  to one of them (to keep his BGP table small) while you want to advertise all detailed prefixes to the others.

In that case, you can create the aggreggate address by the

aggregate-address 10.0.0.0 255.0.0.0 command.

And then you can control by

neighbor ... route-map ... out

commands which prefixes (detailed or summary only) you are advertising to each neighbor.

HTH,

Milan