BGP - Influence incoming traffic

Unanswered Question
May 11th, 2010
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We have two ISP connections for our office. I would like to use one ISP (ISP A) as the primary ISP for both outgoing and incoming traffic. I would like to use the Secondary ISP (ISP B) as a backup only.

I am accepting only default route from both ISPs and advertising only our "/24" prefix to both the ISPs. I use local preference to make sure ISP A is used for all outbound traffic and I use AS prepend on updates to ISP B to ensure that ISP A is used for all incoming traffic.

But, I found that some incoming traffic uses ISP B. I tried increasing the number of prepends and still see some traffic coming through ISP B. I spoke to ISP B and they say that their uplink service provider does not honor AS prepends. Hence this problem.

My question is, has anyone else come across this issue and were you able to solve the problem? (communities, etc. ).


Suresh Krishnan

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bgandhi Tue, 05/11/2010 - 21:51
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Hi Suresh,

You would not be able to control incoming traffic from different ISPs. Reason is even if you prepend AS there is a possibility that the AS which is forwarding the traffic through AS_prepended ISP may find it shorter considering the global scenario. You may increase the AS-prepend count and try.

I assume you have your own AS number. The better option would be to enable conditional advertising using advertise-map and not-exist map.

Hope this would help you to analyze further.




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