This article originally started off life as a record of how I managed to get this working, as a lot of my posts do, but this time it appears I am foiled.
Last week, I had 3 vCenter Servers that appeared to be happily talking to each other in Linked Mode sharing a singe Multi-site SSO domain without any real issues. I had a single-pane-of-glass view of all 3 and I could manage them all from the one client. The reason for the 3 vCenter servers was segregation of LAN and DMZ networks: vCenter001 was in the LAN, vCenter002 sat in DMZ1 and vCenter003 sat in DMZ2.
VMware vSphere Single Sign On (SSO) can be installed in Multi-site mode to support local sign-on to vCenters that you want to be part of the same single sign on domain - for example, if you want to install Linked-Mode and have the advantage of a single pane of glass view, but can't risk using a single SSO instance across the WAN. In other words, from VMware's blog post:
Multisite deployments are where a local replica is maintained at remote sites of the primary vCenter Single Sign-On instance. vCenter Servers are reconfigured to use the local vCenter Single Sign-On service and reduce authentication requests across the WAN. Multisite deployments do drop the support of single pane of glass views unless Linked Mode is utilized and multisite deployments are actually required to maintain Linked Mode configurations where roles, permissions and licenses are replicated between linked vCenter servers. Linked mode will re-enable single pane of glass views across multisite instances.