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Influencing BGP metrics with AS Prepend

Thanks in advance for the recommendations to my issue, I'm running a multihome environment with two different ISP's (A and B) and two different routers, we are PI ASN which I advertise to both ISP's, thought ISP-A and though ISP-B and in reverse each respective secondary IP blocks which I AS prepend to influence the return traffic, typical setup, nothing fancy...everything works fine until I enable iBGP to complete this multihome setup, at that time shows a better BGP metric via ISP-B even with the AS prepend on (which shows in a looking glass router show IP bgp command), I would like my traffic to remain symmetric and by AS prepending I should be accomplishing that, well is not working, I welcome any ideas and suggestions on what else I can try to accomplish my goals.


Influencing BGP metrics with AS Prepend

I need to break this down a bit

RTR A = ADV to ISP A     also ADV w/ AS prepend to ISP A

RTR B = ADV to ISP B    also ADV w/AS prepend to ISP B

without any iBGP what is the output you get from a looking glass?

I would assume it is what you are expecting to see.

In the iBGP set-up, what are you announcing to the neighbors A and B?

So you are saying once iBGP is configured the route is preferred via ISP A

What sounds like is needed to keep the traffic separate is to have some meds placed on the iBGP anouncement to RTR A.  Before anything can be done we need to know what is being advertised to each.

sh ip bgp neigh adv

then also look to see what is being received on the neighbors too.

sh ip bgp neigh recieve-route (or something like that)

New Member

Influencing BGP metrics with AS Prepend

Hi Borman,

You need to increase the number of AS prepending in the BGP AS prepend statement. Let's say you are prepending your own AS 5 times from router B for subnet so increase it to 10 times and see the result. if it don't solve the problem increase the AS prepending it to 15-20 times till the time is best path from router A.

For ex:- Issue is reachability from BGP looking Glass server through ISP-B to your network could be 10 hops and 15 hops through ISP-A so if you prepend own AS for route through ISP-B 4 times still ISP B is best path because it is still 14 hops away as compared to ISP-A which is 15 hops Away.

I hope it answers your question. Please rate the post if you find it useful.



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