BGP specific subnets advetisment

[Vasileios Bouloukos MSc, ITIL, CCIE]

Hi,

I have a question about BGP.

I have two routers with eBGP peering between them.

Lets assume the one router is the R1 and the second router is the R2.

Now, R2 receives a X.X.X.0/23 supernet from R1. R1 has learnt this route (X.X.X.0/23 ) from bgp.

I want R1 to advertise to its ebgp neighbor R2 instead of this supernet X.X.X.X/23  two /32 subnets which belong to the x.x.x.x/23 subnet. If there is a failure and R1 does not have the /23 subnet, then this router will not advertise the specific /32 subnets to R2. I do not want the R1 to generate these two /32 subnet to its bgp topology table but just to advertise these two subnet to the R2 only bgp neighbor.

It is not a problem if the R1 can not filter the /23 subnet and R2 receives both the /23 and the two /32 subnets. Although it is preferred the R2 to not receive the /23 subnets but only the two /32.

Thank you all!

Vasilis

[Edison Ortiz]

I believe this is the feature you are after:

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

Regards,

Edison

[Vasileios Bouloukos MSc, ITIL, CCIE]

Thank you Edison.

I have read this doc.

I do not want to modify the config of the R2 router and i should not inject these two /32 subnets to the R1 BGP topology.

For these reasons I can not configure bgp conditional option.

I am looking for a command applied to the R1 to the bgp peering to R2 and generate the /32 subnets from the /23 subnet.

I have not find anything yet....

[Edison Ortiz]

You don't need to touch R2. I don't understand the requirement to not inject the routes on R1.

Your main post said you wanted that.

[Vasileios Bouloukos MSc, ITIL, CCIE]

I have simulated the setup in GNS.

By using the command

 bgp inject-map ORIGINATE exist-map LEARNED_PATH

to the R1 router which has the /23 subnets (and is advertised to the R2) then adds the /32 subnet to the bgp table.

In  my scenario, because R1 router has BGP peerings with other neighbors, I have then to filter these /32 subnets to the other peering. I want only one eBGP neigbor to learn the /32 subnets.

This is sth that I want to avoid.

I hope to be more clear now.

[Edison Ortiz]

The subnets must be on the BGP table in order to be advertised.

If you don't want other BGP neighbors to receive these subnets, filtering is the only option.

I really don't understand how you want BGP to advertise a route without being entered in the BGP table?

[Vasileios Bouloukos MSc, ITIL, CCIE]

I was inspired by the "aggregate address ...  summary only" command which allows to advertise or supress the more specific subnet along with the summary and I was wondering if I can achieve the setup that i want.

Yes, I agree in all cases the route should be in the BGP table...and since should exist to the routing table then I have to filter the route in order to not receive the rest BGP neighbors.

Maybe in the other world can do this  but not in this world

Thank you Edi.

5 points