The vCenter Server 5.1.x multisite deployment enables a single administrator to administer vCenter Server instances that are deployed on geographically dispersed sites in Linked Mode. when you upgrade to vCenter Server 5.5, the vCenter Single Sign-On deployment changes.
About this task
In vCenter Server 5.5, each vCenter Single Sign-On instance uses the default identity source, and can use other identity sources if the domain is included when a user logs in.
There are no components in the vSphere suite that communicate with multiple vCenter Single Sign-On servers. Each vSphere component should be configured to communicate with its local vCenter Single Sign-On instance for faster access.
vCenter Server 5.5 supports connection between vCenter Server and vCenter Server components by IP address only if the IP address is IPv4-compliant. To connect to a vCenter Server system in an IPv6 environment, you must use the fully qualified domain name (FQDN) or host name of the vCenter Server. The best practice is to use the FQDN, which works in all cases, instead of the IP address, which can change if assigned by DHCP.
Review the topics in Preparing for the Upgrade to vCenter Server.
When you upgrade from an existing multisite Single Sign-On deployment of vCenter Server, to maintain Linked Mode functionality you must upgrade all Single Sign-On instances to the same version and manually resynchronize all Single Sign-On instances. See Knowledge Base articles http://kb.vmware.com/kb/2042849 and http://kb.vmware.com/kb/2038677.