why is the router not sending a full BGP table

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Mar 2nd, 2009
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I have two routers, A and B.

router A has got a link to service provider X, and receives a full bgp table 274K.

Router B has got a link to service provider Y and receives as well a full BGP table.

there is an IBGP session between A and B.

my question is, why router A does not send the full BGP table received from provider X, to router B.

At this time router A is only sending 212K routes. router B as well does not send a full BGP table to A, only 212k.

I can't see any route filtering between A and B. is there a way I can double check which route are not exchanged.


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Harold Ritter Mon, 03/02/2009 - 10:27
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Router A doesn't send its routes to router B probably because it considers router B as the best path for these routes.


kayih Mon, 03/02/2009 - 10:53
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uhmm, but before the routers decide who has got the best path, they should exchange their prefix from each other and put it in the BGP table. Then the best route get installed in the routing table.

I don't think the routers exchange their best path, but the network they can reach, so it's the receiving router who decides which route to use

or I'm missing something ??

Mohamad Qayoom Mon, 03/02/2009 - 13:03
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Can you do the following and see if you are recieving same number of prefixes on router B that are advertised by router A?

On Router A

sh ip bgp neighbors b.b.b.b advertised-routes

On Router B

sh ip bgp neighbors a.a.a.a received-routes

Harold Ritter Tue, 03/03/2009 - 06:18
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It depends on timing. If router A receives the routes from iBGP, then receives them from eBGP and select the iBGP over the eBGP due to some attributes, router A never send its routes to router B.

Another scenario would be that both router A and B receive all of the Internet routes from their respective eBGP session and then advertise them all to each other. If router A then prefers the route coming from iBGP (again according to some attribute value being preferred), router A sends a withdraw message for all the routes.



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