customizing incoming Internet BGP routing table

Unanswered Question
Aug 27th, 2009


I have 2 BGP links at my gateway router.

Link A is 10 Mb bandwidth and receive full internet routing table from ISP A. Meanwhile link B is 20Mb bandwidth and receive summarize routing table from ISP B. I want to make Link B as primary and have configured higher local preference for Link B.

The problem is, most of the packet go to Link A because it has specific prefix due to internet full routing table.

Is there a way to tweak this?

I have ask ISP B to give full routing table but they are not cooperating.

I also try to ask ISP A to give only summarize routes but they are not cooperating also.

I'm thinking to filter the full routing table from ISP A, so that it just include the network prefix from ISP A with default route. So most of the packet will go through link B because it has higher local preference. Can you show me how to do this?

Would it affect the router performance?

For your info, my gateway router is Cat 6509.

Any advice?

Thank you

I have this problem too.
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nizamismail Thu, 08/27/2009 - 20:00

Hi Guru Prasad,

Thank you very much for the links. It is very helpful. I get the idea on how to do it.

I have one question. If I configure my router to only receive routes originating from AS 4; does the default route from AS 4 will include in my routing table or the default route is filter out?

ralexandrov Fri, 08/28/2009 - 04:37

If you have 2 separate conns to ISPs I can't understand why you are not using just local preference attribute?

Set it to higher value via route-map configuration, so you'll be having both the conns on and redundant setup.

If you do filtering of the routes, as you're trying to do you'll never get automated backup.

c.captari Thu, 08/27/2009 - 22:29

Assume that your ISP AS is 4.

router bgp 1

neighbor remote-as 4

neighbor route-map foo in

route-map foo permit 10

match as-path 1

ip as-path access-list 1 permit ^4$

This only permits routes originating in AS4 all other are denied.

Now you've got yourself a partial routing table from AS4, but bear in mind that you are missing a default route, so you need to speak with AS4 so that along with the full routing table they send you a default route as well. This should be pretty straight forward and they should accept this as it's very easy to configure.


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