There are several considerations to take into account before you configure a Linked Mode group.

Each vCenter Server user sees the vCenter Server instances on which they have valid permissions.

When first setting up your vCenter Server Linked Mode group, you must install the first vCenter Server as a standalone instance because you do not yet have a remote vCenter Server machine to join. Subsequent vCenter Server instances can join the first vCenter Server or other vCenter Server instances that have joined the Linked Mode group.

If you are joining a vCenter Server to a standalone instance that is not part of a domain, you must add the standalone instance to a domain and add a domain user as an administrator.

The vCenter Server instances in a Linked Mode group do not need to have the same domain user login. The instances can run under different domain accounts. By default, they run as the LocalSystem account of the machine on which they are running, which means they are different accounts.

During vCenter Server installation, if you enter an IP address for the remote instance of vCenter Server, the installer converts it into a fully qualified domain name.

You cannot join a Linked Mode group during the upgrade procedure when you are upgrading from VirtualCenter 25 to vCenter Server 4.1. You can join after the upgrade to vCenter Server is complete. See the vSphere Upgrade Guide.