Multi-Master is simply setting more than one server as “Grid Primary”. In general, you set one to “Grid Primary” and the others to “Grid Secondary”. If you set multiple to Grid Primary, any Grid Secondary will, at configuration or editing in the name server group, allow you to set the preferred primary via automatic selection or manual process.

If you query any Grid Primary for a multi-master zone, it will always return itself as the SOA. The secondaries will refer to a preferred Grid Primary but, if it is removed, they will start referring to the next Grid Primary.

There are performance implications of being multi-master. Consider multi-master only in scenarios where things are highly dispersed (e.g. across continents), where you have clients doing updates to a zone that spans the regions. Also consider it where GM/GMC are the DNS servers and you want to allow for DNS updates when GM dies and GMC hasn't been promoted yet. Multimaster is okay in smaller scenarios.