subnet ignored while creating aggregate-address in BGP

Unanswered Question
Nov 5th, 2009

Hi,

does anybody know a way how to ignore one subnet when creating an aggregate-address in BGP?

I need to create aggregate-address 172.16.0.0/12 when receiving some more specific subnets via IGP.

But I've got a directly connected 172.30.1.0/24. So the aggregate-address 172.16.0.0/12 is created all the time.

I have not found any aggregate-address command options how to handle this :-(

Thanks,

Milan

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Giuseppe Larosa Thu, 11/05/2009 - 05:03

Hello Milan,

I'm afraid you are right in command reference there is an advertise-map option but it can be used only to modify some BGP attributes of the aggregate advertisement.

see

http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_bgp1.html#wp1011467

so or you able to re-address the link with a subnet that is in a different major network or you should look at a different solution to generate the prefix.

I haven't tried, but I wonder if using reliable static routing with object tracking could be used for a static to null0.

so that a BGP network command could be removed if the static to null0 is removed when tracked objects fail.

see

http://www.cisco.com/en/US/docs/ios/12_3/12_3x/12_3xe/feature/guide/dbackupx.html#wp1071672

this should be tested in a lab.

or BGP conditional advertising if there is a way to use it for this scenario.

Hope to help

Giuseppe

milan.kulik Thu, 11/05/2009 - 06:40

Hi Giuseppe,

thanks for confirmation I was not missing any simple way of using just aggregate-address command.

I was still hoping my understanding of advertise-map option was wrong and it could somehow exclude particular subnets from the process of the aggregated prefix creation.

(Which is IMHO a good idea and Cisco might have included this to the IOS.)

Static routing with object tracking might be the way, but I don't want to use Ping as the tracking tool.

I remember we were using OSPF routing table for HSRP tracking in the past somewhere.

I need to search my archive.

It might be possible to use one prefix presence in BGP table as a track for static route possibly?

So far I was thinking about conditional advertising, but my design is so complex I was not able to find a solution fulfilling all requests (as the same subnet received from a peering neighbor must be accepted as preferred under some conditions).

But another idea just came to my mind:

BGP conditional advertising with two /13 replacing one /12.

As longest match wins, this might work!

I'm going to test this idea in my lab.

Sometimes just trying to explain your problem to other guys brings you ideas :-))

Thanks,

Milan

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