in our environment we have two vCenter at different site in linked mode with muti-site same SSO with vSphere 5.5 U3. What is the best practice to upgrade vSphere 6?
Can we have external PSC on dedicated system and upgrade vCenter, then join the both vcenter to same PSC??
Technically speaking, you probably could connect both vCenters to the same PSC. However it is not recommended, since you have vCenters in multiple sites (A vCenter authentication and certificate management will depend on a WAN link - which isn't good).
As tdubb123 said, it is recommended to configure a PSC for each site and join them to the same SSO Domain - and they will use Enhanced Linked Mode. Also it is not recommended to have a embedded PSC talking to an external (Not scalable).
So you are going to have:
This is the procedure I've been using (assuming you have embedded SSO):
1. Break Linked Mode (vSphere 5.5 Documentation Center)
2. Upgrade the vCenters to 6.0
3. Create a an external PSC on site 1 and connect it to the SSO domain that is already used.
4.Migrate from the SSO of 5.5 to the PSC (VMware KB: Migrating embedded VMware vCenter Single Sign-On to an external VMware vCenter Single Sig...)
5.Repeat the process on the other site and connect to the SSO domain (looks similar to this vSphere 6.0 Documentation Center)(make sure to follow this link to reconfigure the sso vSphere 6.0 Documentation Center)
Hope this helps.