Manipulate outgoing BGP Path selection.

Unanswered Question
Dec 26th, 2007

Good Afternoon,

I have two Cisco 7206 VXR routers connected to two different upstream service providers using BGP. Both connections are OC3 and at this time we are utilizing 45% incoming traffic on both service providers, however we are using 30% outgoing on SP A and 0% on SP B. I was able to balance out the incoming through AS prepends etc... Anyways - this is not a major issue for me right now but my SP B just became multihomed with my SP A. So now when outgoing traffic selects its path to internet destination its sees 95% of the routes in the BGP topology table with one extra AS through SP B. I have talked to Cisco and they say I can't influence this I can only influence how I send my routes to my SP's. I just want to make sure that there is no way to accomplish this so that in the event that SP A ever becomes nearly saturated I can start influencing outbound traffic to take the less desirable AS path.

So the jist is how can I influence the routes that I am receiving to possibly remove the first AS or prepend an additional AS for incoming updates?

I know that I can select to have partial updates from SP if necessary or create fun route maps and specify certain networks to use one SP and others to use the second SP, but I just want to know if I can do something to the SP B as a hole to modify its number of AS hops and allow it to use some other metric to make decisions.

Thanks,

David

I have this problem too.
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Richard Burts Wed, 12/26/2007 - 10:56

David

It is true that you can not influence how many hops appear in advertisements from SP B. But that does not mean that you can not choose to prefer some routes from SP B. It is slightly complicated and will take some work on your part, but it is doable. By assigning a better local-preference to some routes to SP B, you can make those routes be preferred. So you would need to identify some routes (you probably do not want to prefer all routes from SP B) and give them the local pref. Or you might ask SP B to send you a subset of the complete routing table and a default route and you could local-preference for these.

So I do not think that you can do anything with SP B to reduce the number of hops. But you can use local preference to change the metric and make some different choices of outbound route.

HTH

Rick

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