You can join multiple vCenter Server systems using vCenter Linked Mode (introduced in vSphere 4.0) to build a single pane of glass and share information between vCenter instances, such as views, and manage the inventories of all the vCenter Server systems that are linked. With vSphere 6.0, a new vCenter Enhanced Linked Mode (ELM) was introduced to replace the existing Linked Mode capability that was based on Microsoft ADAM technology. In this way, it was finally possible to have Linked Mode capability for both Windows-based vCenter Server as well as the vCSA, with new features and functions, such as the cross-vCenter vMotion.
With ELM, it is possible to use a single SSO domain for all vCenters with single-pane-of-glass management (HTML5 client is also supported), with a maximum of 15 vCenters (with vSphere 6.5 U1). A PSC must be deployed externally; embedded vCenter Server is not supported. Roles, global permissions, licenses, certificates, vSphere tags, and VM storage policies are automatically replicated across all vCenter instances.
You have to configure ELM during installation of PSC and vCenter Server; post-deployment is not supported.
But with the new VMware Cloud on AWS (VMC) offering, there is also a new Hybrid Linked Mode (HLM), with a completely different implementation and some differences from ELM:
- SSO domains will be different between on-premises and VMC; however, it is a 1:1 relationship.
- Instead of a two-way trust, a one-way trust is established, where the cloud part trusts the on-premises vCenter Server and data is synced unidirectional from on-premises to the cloud.
- It can be configured at any point in the on-premises vCenter Server, not only during the initial installation.
- Actually, it requires that the on-premises vCenter Server is an embedded deployment; an external PSC is not supported. It will be interesting to see whether it will be changed in the future.
For more information about the differences between ELM and HLM, see http://www.virtuallyghetto.com/2017/09/enhanced-linked-mode-elm-vs-hybrid-linked-mode-hlm.html.