BGP entrance influence

Unanswered Question

I have an ASN and 2 Internet providers. One provider is 3Mb, and the other 10Mb. Because my ISP controlling the 3Mb link has a bigger network than the other, most sources are preferring to take the 3Mb path over the 10. Is there anything I can do with the BGP configuration to influence how other people come into my network, and prefer the 10Mb over the 3Mb link? Thanks Netpros.

I have this problem too.
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Kevin Dorrell Thu, 10/25/2007 - 05:24

There is no sure-fire way of doing this. You could try pre-pending your own AS in front of your advertisments through the 3Mbps provider. That way some of the remote ASs will get a longer AS path though the 3 Mbps link and might be more inclined to come through the 10 Mbps provider. But it's a bit touch-and-go, and there is no guarantee that the provider wont strip the extra AS prepends.

Are you getting a large routing table from both? If you look at the AS path to your biggest user prefixes, is the AS path from the 3 Mbps link much shorter than the AS path from the 10 Mbps provider? Or are they both just giving you defaults/summaries?

Kevin Dorrell


paul.matthews Thu, 10/25/2007 - 23:38

As you are going to two ISPs, the only real option available to you is AS_PATH prepending. The "long distance" metric is basically AS_PATH length. You can add your own AS number to the AS_PATH to make it longer on the 3M link.

Depending upon what you want to acheive, you can add the AS_PATH once and look at the effect on the traffic, then add it again to try to get an approximate 3:1 balance (please note AS_PATH length is a bit of a blunt instrument, you may have to make do with a less balanced ratio.

If you just want traffic to generally prefer the 10M you could start by adding your AS No about ten times...


saimbt Mon, 10/29/2007 - 05:47

One more thought.

If your ISP1 (3 Mbps) and ISP2 (10 Mbps) have a peering relationship, there are chances that the local Preference for your network on the ISP2 is lesser than the local preference between ISP1 and ISP2. In this case inspite of having AS Prepend, most of the routes falling in ISP2 would give the routes to ISP1.

In this case either make the local preference for ISP1 and ISP2 default (100) or increase the local preference for your network on ISP2 greater.

Did i confuse you?



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