subnet advertisement with different BGP peer

[Terence Payet]

Hi expert,

This might be a stupid a question. But just bear with me. Am actually a noob in BGP, so your help is greatly appreciated.

I have the below scenario:

Three providers

P1 subnets: 1.1.1.0/24, 2.2.2.0/24

P2 subnets:  3.3.3.0/24 4.4.4.0/24

P3 subnets: 5.5.5.0/24 6.6.6.0/24

Lets say i want to advertise subnets 2.2.2.0/24 from P1 to P2 and advertise P3 5.5.5.0/24 to P2.

Is this possible?

How will i go about the config?

Thanks to advise.

Steve

[John Blakley · Accepted solution]

Steve,

It depends on how you're peered up and if you have any filtering between you. By default, BGP will advertise anything that's in its table. For example, if you have:

P1 -- P2 -- P3

And you advertise 1.1.1.0/24 from P1 to P2, P2 will advertise to P3. There are caveats though. That's with an ebgp peering (Different AS numbers). If you have all of these in ibgp (same AS number), P1 will advertise to P2, but P2 will not advertise to P3 because it was an ibgp learned route. In that scenario, you'd have to have a full mesh of peerings or use confederations/route reflectors.

HTH,

John

[Richard Burts · Accepted solution]

Steve

There are several filtering mechanisms available in BGP that allow you to control what you advertise to specific peers (and to control what you learn from specific peers) so what you ask should certainly be possible. The most common mechanism is a route map which you can apply outbound or inbound on a per neighbor basis. You could also look at using prefix lists or distribute lists to do the filtering.

HTH

Rick

[Terence Payet]

Thanks Rick

[Terence Payet]

Many thanks for your response John.

regards,

Steve