BGP de-aggregation

[Kevin Dorrell]

What is the best way to de-aggregate a prefix in BGP?  By which I mean I have a /22 prefix in my network, let us say 192.0.96.0/22, but my BGP peer will only accept /24 routes from me.  So I need to split it up into 192.0.96.0/24, 192.0.97.0/24, 192.0.98.0/24, and 192.0.99.0/24.

I know how to use aggregates to combine routes, so that if any fragment of the aggregate is in my BGP, then the aggregate gets advertised.  But can I do it the other way: that is declare some /24 aggregates, and if my /22 exists then the /24s get advertised?  If not, can you suggest another way of doing it?

Thanks in advance

Kevin Dorrell

Luxembourg

[milan.kulik · Accepted solution]

Hi Kevin,

BGP Conditional Route Injection should be a solution for you.

See

http://www.cisco.com/en/US/customer/docs/ios/12_2t/12_2t4/feature/guide/ftbgpri.html

for details.

BR,

Milan

[Kevin Dorrell]

Hi Milan,

Thanks for the answer.  How frustrating!  The feature is exactly what I need, and my switches do support it ... but apparently not in a vrf !

At the moment I am originating the /24 prefixes using static routes and network statements, but that is not ideal precisely because they are static routes.  Back to the drawing board.

But your answer to the question I posed is correct, so thank you for the idea.

Kevin DORRELL

Luxembourg

[milan.kulik]

Hi Kevin,

I suppose you are receiving the /22 prefix via some IGP from your network?

So wouln'd it be possible to configure the /24 static routes somewhere deeper in your network where it would not create any redundancy problem?

BR,

Milan

[Kevin Dorrell]

I suppose I could.  The /22 is actually an oversized data centre VLAN, spanning two sites.  - histerical historical reasons.  The VLAN is served by SVIs on two L3 switches in HSRP, each of which advertises the /22 into OSPF.

So, on the data centre switches, as well as the connected /22 routes for the VLAN SVI, I could configure four /24 statics pointing to the VLAN SVI, and redistribute those into the OSPF.

I am thinking aloud here, and I wonder if I have painted myself into a corner.  The area that carries the VLAN is a totally stubby, so redistribution may be out of the question.  Or is it?  The data centre switches are also ABRs, so I guess the re-distribution would be into area 0, which would be OK wouldn't it?  I should try it in the lab first.

Thanks for the reply.

Best regards

Kevin

[kradjesh13]

Hi Kevin

I am using suppress-map to de-aggregate and to inject specific routes in the routing table. If you need further information please let me know I have a scenario handy.

Thanks