iBGP Question

Unanswered Question
Mar 6th, 2007

Can anybody please explain why routes learned from an iBGP peer aren't propagated to other iBGP peers (hence the need for full mesh or reflectors)?

I have this problem too.
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Jon Marshall Tue, 03/06/2007 - 00:54


They aren't propogated under normal circumstances to avoid routing loops. EBGP can use the AS Path in the routing update to ensure that there are not routing loops ie. if the router sees it's own AS in the AS Path it knows there is a potential routing loop and so will not accept the route under normal circumstances.

Because the AS Path does not get changed when routes are exchanged between IBGP peers the router has no way of knowing if there is a potential routing loop.



Christopher007 Tue, 03/06/2007 - 01:05

Ok, this makes sense. Since an IGP is responsible for forwarding the updates between the iBGP peers, wouldn't it be the IGPs responsiblity to ensure no loops to get to the next AS hop. The only problem I can see with iBGP forwarding updates to other iBGP neighbors is possibly sub-optimal routing to get to the nearest "edge" BGP peer.

Jon Marshall Tue, 03/06/2007 - 01:13

It can't really be the IGP's responsibility because it is just another bit of IP traffic to the IGP. And the IBGP speaking router stores the updates in a separate table - the BGP table. Only then will it decude whether to install it in the IGP routing table.

Before this the IGP is used merely to transport some tcp traffic between 2 routers.




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