BGP Cluster Id

[msaltunsaray]

Hi expert,

I ve got a question about bgp cluster-id.

I just would like to learn what is the difference between having cluster-id or not.

I am recently configuring two new RRs for our network and curious whether I should configure cluster-id!

Could you please refer me what will happen if I configure it or not?

Thanks

[Georg Pauwen]

Hello,

what are you after ? I don't think you can have a route reflector WITHOUT a cluster ID, as the cluster ID is appended to reflected routes.

If you have redundant RRs with the same cluster ID, they will discard each other's routes. If they have different cluster IDs, they will accept each other's routes.

[msaltunsaray]

Hello,

Actually yes you can. So I have already configured two.

I just want to learn what is the exact benefit of creating cluster-id?

I know the difference of using same or different cluster-id!

Thanks

[Georg Pauwen]

Hello,

out of curiosity: how did you remove the cluster ID from the route reflector ? If you don't specify one, it is automatically generated from the BGP router ID....

[Giuseppe Larosa · Accepted solution]

Hello msaltunsaray,

>> I just want to learn what is the exact benefit of creating cluster-id?

if you have two BGP Route Reflector Servers serving the same set of BGP clients on the same address families you can set the same BGP cluster-id on them so that they will be able to silently discard routes having the common cluster-id in the Cluster-List attribute.

This saves some memory resources and avoids to have replications between Route Reflector Servers.

However, from a fault tolerance point of view configuring the same cluster-id on the two Route Reflector Servers does not cover a specific double fault when client X has the iBGP session with RRS1 enabled but the one with RRS2 failed (for a neighbor shutdown command for example) and another client Y has iBGP session with RRS2 enabled and the one to RRS1 disabled.

In this case if the two RRS RRS1 and RRS2 use the same cluster-id routes originated by client X cannot reach client Y and viceversa.

So in some service provider network designs the use of a common cluster-id has been removed.

Two levels of BGP route reflection are possible to minimize the iBGP full mesh between them in very large networks.

To be noted in other BGP implementations of other vendors (like Juniper JUNOS) the configuration of the cluster-id is mandatory in BGP route reflector servers, but it can be equal to the BGP router-id.

Cisco IOS provides the way to set the cluster-id equal to BGP router-id if not using the command.

This has to be considered an implementation choice as noted above about other vendors.

Hope to help

Giuseppe