BGP Routes not being advertised from iBGP to eBGP

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Feb 5th, 2010
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Greetings,


I apologize if this question has been answered before, but I'm having a configuration/design issue with BGP.


My company has purchased another company, and as such we have connected a trunk and want to begin advertising their /22 across our backbones (we are a ISP).


This is how the BGP sessions are currently set up


ASN #1 (Router) ----> ASN #2 (Route Server) ----> ASN#2 (Backbone Router) --- ASN #3 (Backbone Provider)


I've designated the session from the route server and the backbone as route-reflector-client (from the route server side only).


The /22 goes all the way to the Backbone Router, but the backbone router does not advertise the /22 to the Backbone Provider (that it receives from the Route Server).


Any ideas? I'm not very clear with how route reflection should be set up. I plan on going through my Cisco books but if you could provide any advice, it would be greatly appreciated.

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marikakis Fri, 02/05/2010 - 22:35
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Can you check if synchronization is enabled on the Backbone Router (and in general)? If synchronization is enabled in ASN#2, border routers won't pass routes learned via iBGP to eBGP peers, unless routes are also in the IGP. The recommended solution is to disable synchronization (you need to make sure you have full iBGP in the transit path or else that routers in the transit path will ultimately find their way to the proper edge router).

milan.kulik Sun, 02/07/2010 - 03:01
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Hi,


IMHO, you don't need route reflectors in your topology.

"a BGP speaker does not advertise a route learned from another IBGP speaker to a third IBGP speaker."

see http://www.cisco.com/en/US/docs/internetworking/case/studies/icsbgp4.html#wp7158


But this is not your case!

You don't have three routers in the same AS.


As Maria pointed out, synchronizastion (http://www.cisco.com/en/US/docs/internetworking/case/studies/icsbgp4.html#wp19382)  might be the cause of your problem.


BR,

Milan

marikakis Sun, 02/07/2010 - 03:29
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Hi Milan,


I agree with you. If the topology is as simple as was initially described, then I doubt if the issue has anything to do with the way an RR is set up. This logical topology would work even without route reflection. Issue could be also due to some filter in the OUT direction between Backbone Router and Backbone Provider, but for some reason this case immediately struck me as a synchronization issue.


Kind Regards,

Maria

fortino.brandon Mon, 02/08/2010 - 06:17
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Actually synchronization is turned off in all of the routers mentioned. Which is why I'm stumped. The backbone router is clearly receiving the /22 from the route server and is not advertising it down the backbone pipe. We are using access lists to filter the updates but the block is on the permit lists for all ACLs concerned.

marikakis Mon, 02/08/2010 - 06:28
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1. Have you softly cleared the session towards the backbone provider after updating the filters?

2. Do the incoming filters of the backbone provider allow the prefix advertised?

3. Are you sure the mask of the route is the correct one? Can you post the filters from both sides of the session?

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