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BGP4 on dissimilar edge routers...


Have a customer that is going to be setting up redundant internet circuits in the relatively near future. They currently have a t1 that is handled by a 2611. They are adding a 10Mbit circuit that is going to be handled by a new 3640. They will be announcing routes to both isp's via BGP4 (primarily for failover in this case), but they would like to get the benefit of good (and hopefully symmetric) outbound path selection.

Because of the discrepancy in bandwidth and latency in the circuits, the primary preference for the t1 outbound would be for other customers of the same (relatively large) ISP. If we just stuck with ISP routes for the 2611, would there be any benefit to requesting full internet routes on the 3640? If we don't pull in a full routing table on the 3640, what is the best way to indicate that that circuit is preferred in the event neither router has a route for the destination?

Any pointers to examples/case studies are of course appreciated.



Re: BGP4 on dissimilar edge routers...

This is pretty complex, actually.... Since you are already running bgp with the provider, it's a bit easier, though. There are two things to consider--exactly how you want to split the traffic, and how to keep the inbound links doing the right thing, too. On the outbound side, it sounds like the best thing to do is to accept the deafult from the primary, and the default and partials from the backup. If you accept just the routes that are local to the backup from them, and make their default look a lot worse than the primary, then you will get what you describe above.

On the inbound side, one of your service providers is going to be summarizing, while the other isn't. You'll have to ask the one that is summarizing to poke a hole through their aggregate.

Email me if you want more detail than this....


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