If you are setting up A as the Hub, then when you save and that ILS Cluster Registration pop-up window comes up leave the Registration Server blank and click "OK". When you set up the spoke clusters, however, when that ILS Cluster Registration pop-up window comes up you will enter the IP address of the hub cluster.
If you are setting up A and B as hubs that will talk to each other, and setting up additional clusters as spoke clusters, then when the ILS Cluster Registration pop-up window comes up on A and B you will want to point them to each other as registration servers. As before, for the spoke clusters you will point them to their hub cluster for registration.
For Hub clusters, the fact that you can only enter one IP as the registration server does not mean that a hub will communicate with only that other hub. For an environment where you have three or more hubs, any clusters identified as Hubs will learn about all other hubs and will directly exchange information with all other Hubs.
Spoke clusters only exchange information with their hub cluster.
Hubs will exchange information with each other on a mesh basis (so two or three or more hubs will all intercommunicate). Hub clusters will also exchange information with any spokes registered to it. Hub clusters will then share that information with other hub clusters, which will in turn share that information with their spoke clusters. This means that eventually all clusters will have information (URIs, GDPR Patterns, etc.) advertised by all other clusters.
It is very similar to how routing protocols work with regards to the sharing of information.
FYI: If you have enough clusters where you are thinking of having two hub clusters, you probably should consider CUCM Session Manager Edition as an alternate architecture. It is specifically designed for many-cluster environments.
Let me know if you have additional questions.
Maren
