Hi @Anandu 

This both veeam servers normally will not work together when you are doing Veeam Replicas or CDP. So you can do that. 
I know about some scenarios where a second vbr server does the replication with imported backups (with mapped backup job somehow), then you would/should to have them on the same version.

Please Read the release notes of V11a to find out what is different with V11a.

https://www.veeam.com/kb4215

Even if I performed a patch update in v11?

If you are doing Veeam CDP, only one VBR is involved in the process. So the second vbr can completely ignored.

For doing Veeam Replicas, if I remember correctly, there was an method to use another vbr to import the backup files, map them to dummy jobs and create replicas from this backups. Then both vbrs must be on the same level. An older vbr server can not import backup files from newer vbr server.

are you talking about replica seeding? If so without replica seeding, can we initiate replication between two different veeam versions?

There is no replication between two veeam servers. Veeam only replicates between hypervisors.

Can you tell us a little bit more what you are trying to achieve?

I want to initiate replication and CDP between two sites, one site has installed v11a, the other v11. since both sites have different versions, does that cause any effect to replication or CDP?

You can safely ignore the second vbr on the second site. It will not be used in what you are trying todo.

So zero effects on CDP or Replication, as long you are using only one of them to configure cdp.
Replication can be used with both, but better not for the same vms.

Okay, Thank you

On recommendation from my side. When replicating to a different site, use the VBR server at the target site for the replication of your production/source VMs. So if the site goes down, the VBR server is still available for failover/fallback.