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BGP specific subnets advetisment


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!


Everyone's tags (4)
Hall of Fame Super Bronze

BGP specific subnets advetisment

I believe this is the feature you are after:



BGP specific subnets advetisment

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....

Hall of Fame Super Bronze

Re: BGP specific subnets advetisment

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.

Re: BGP specific subnets advetisment

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.

Hall of Fame Super Bronze

BGP specific subnets advetisment

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?

BGP specific subnets advetisment

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