In vSphere 6.0, the PSC introduces multi-master replication. It is important to understand that the default replication topology is one-to-one, which may not be immediately intuitive.
For example, a site with three sites, each with a single PSC, connected via Enhanced Linked Mode, may follow this replication topology:
In this scenario, Site 1 and Site 3 do not replicate directly, only via proxy with site 2.
Here’s another topology that might “happen”, if you aren’t taking too much care about how you are deploying this:
In this scenario, Site 2 and Site 3 both replicate directly to Site 1, but not directly between Site 2 and Site 3.
In both of these scenarios, all three sites will converge on the same replication set. However, a single site failure will prevent replication between two other sites. This design concern becomes more critical in larger environments with more sites.
Recommendation – Use a Ring
For our larger customers we have used a ring topology to improve the design. Using a ring topology is recommended in the VMware SDDC validated design. Following our three site example, a ring topology would look like this:
So, how is the replication topology determined? When you deploy each subsequent PSC after the first, during the install wizard or in the install script you must specify an existing PSC to connect for enhanced linked mode. A replication relationship will be formed between the target PSC and the PSC you are currently deploying.
How To Deploy a Ring Topology
Ring is simple – it is just a straight line, with the last deployed PSC having an additional replication relationship to the first node in the ring. Carefully planning the order of PSC deployments will allow you to build the “straight line” replication topology (see the first image). Once that’s done, log on to the last PSC via SSH and add a new 2-way replication relationship to the first PSC:
vdcrepadmin -f createagreement -2 -h pscsite3.example.local -H pscsite1.example.local -u administrator -w Administrator_Password
For more details, see this incredibly helpful article: https://kb.vmware.com/kb/2127057