11-05-2009 03:22 AM - edited 03-05-2019 06:41 AM
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
11-05-2009 05:03 AM
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
11-05-2009 06:40 AM
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
Discover and save your favorite ideas. Come back to expert answers, step-by-step guides, recent topics, and more.
New here? Get started with these tips. How to use Community New member guide