Multi-Home BGP question

[Arthur Kant]

Hey gang here is an easy question I hope about BGP.

Given the following network:

I have a two Internet edge routers (Router A and Router B) both pulling multiple full tables from transit providers via ebgp.  Both routers are also connected together and run ibgp between each other.

Router A has Transit Provider 1 connected and Router B has Transit Provider 3 & 4 connected.

If I do a show ip bgp 1.1.1.1/8 on Router A I see that the best path to the prefix is through Transit Provider 1 and nothing else

If I do a show ip bgp 1.1.1.1/8 on Router B I see the best path to the prefix through Transit Provider 1 with next-hope of Router A.  I then also see other paths via Transit provider 3 & 4.

My questions:

1) Why do I not see the paths via Transit provider 3 & 4 on Router A ?   Is it because Router B is not advertising it to me because it is learning the best path from Router A ?  

2) If Router A lost its connection to Transit 1 would happen?  The route would disappear and thus not be advertised to Router B would that cause Router B to then send over its best path for Router A?

Thanks!

Kenny

[vivek srivastava]

1) need some more information

on Router B - "sh ip bgp neigh <Transit 3> received-routes

on Router B - "sh ip bgp neigh <Transit 4> received-routes

on Router B - "show ip bgp 1.1.1.1/8" [complete output]

2) If Router A to Transit 1 lost, then in ideal scenario you should see best path on Router B for 1.1.1.1/8 via Transit 3 or Transit 4

Also my doubt, are you able to see 1.1.1.1/8 or 1.0.0.0/8 ?? have you configured "no auto-summary" ??

Vivek

[Joseph W. Doherty]

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Posting

It also might be, 1.1.1.1/8 has one less AS hop via Transit Provider 1 than via Transit Providers 3 and 4.  If prefix has some number of AS hops from each Transit Provider, your local router will prefer it.

[Arthur Kant]

Gentlemen thank you for the replies thus far.  I think the reason for this behavior is due to BGP rules only advertising the best path it knows to another peer.  Since Router B is not has its best path to the AS through Router A it does not have eligible path to send to Router A.  I also neglected to say that both router A&B are in a conferdation .