Advertising More Specific Prefix while using Aggregate-address command

[Kandarp Patel]

Hello There,

We are using "aggregate-address 192.168.0.0/16 (Just an example) summary-only" to advertise summary prefixes to ISP. 

Is it possible to manage inbound traffic via specific ISP by doing AS-Path prepend for more specific subnet i.e. 192.168.0.0/24 without impacting Summary addresses?

Thank you

[MHM Cisco World]

I run small lab 
and Yes you can 
My lab 
R1-R2 
R1 I config agg. prefix with summary-only 
then I use route-map OUT to R2 neighbor match agg. prefix and set as-path prepend and it work. 

[Harold Ritter]

Hi @Kandarp Patel ,

AS path prepend does not always work, because service providers will often set and use the local preference to select the best path in their network.

What I would recommend in your scenario is to advertise the more specific prefixes one way or the other and advertise the summary to both service providers. an example would be;

- Advertise 192.168.0.0/17 and 192.168.0.0/16 to provider A 

- Advertise 192.168.128.0/17 and 192.168.0.0/16 to provider B

This way, traffic to the more specific 192.168.0.0/17 would always come through provider A, unless the link to provider A goes down. The same would apply 192.168.128.0/17 through provider B. This is just an example and you could be more granular depending of the prefix you own.

This is in my view a safer way to influence traffic coming from the Internet into your AS. This would also provide redundancy.

Regards,

[Kandarp Patel · Accepted solution]

Thank you @Harold Ritter and @MHM Cisco World for your input. 

I was able to find this solution from my colleague which is "Suppress-route". I tested it and is exactly what we wanted to achieve. 

here is the sample configuration to be configured on either ISP facing router from which we want to advertise more specific route to.

route-policy Test_Route_Suppress

 if destination in (192.168.0.0/24) then
    pass
  else
    suppress-route
  endif
end-policy

router bgp 65010
address-family ipv4 unicast
  no aggregate-address 192.168.0.0/16 summary-only
  aggregate-address 192.168.0.0/24 route-policy Test_Route_Suppress