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BGP with 2 ISP's and 2 routers limit prefixes

We have 2 ISP's connected to 2 ASR routers. iBGP is configured between the 2. We get partial routes from each ISP. As soon as we connect the iBGP session, our 2 routers exchange each other ISP prefixes. This causes one router to cease the BGP with the upstream ISP as it runs out of memory. What is the easiest way limit the number of prefixes that get exchanged.

THX

3 REPLIES

BGP with 2 ISP's and 2 routers limit prefixes

You can limit the amount of prefixes with:

neighbor maximum-prefix <1-2147483647>

HTH,

John

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HTH, John *** Please rate all useful posts ***
Community Member

Re: BGP with 2 ISP's and 2 routers limit prefixes

That setting will close the bgp session with the ISP......this is what we don't want.

Sent from Cisco Technical Support iPad App

Re: BGP with 2 ISP's and 2 routers limit prefixes

Hi,

you need to know why you are getting just partial prefixes from each ISP.

Then you could find some prefixes you are sure would not be used by the other router and filter them out from the iBGP advertisement.

For example:

You know your router A is peering to an ISP A which owns AS numbers x,y,z.

So when configuring iBGP peering on your router B, you can filter out all prefixes containing x, y, or z in the AS_PATH and not send them to router A, as it's receiveing them from its ISP A directly probably.

You could extend  this approach to all ISP A directly neighboring AS numbers, e.g.

Or you could use some other rule to filter the prefixes - based on the reason which partial prefixes you are receiving from each ISP.

HTH,

Milan

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