I used to install vCenter 5.1 using the simple install method. I like it easy. Then… I had to configure vCenter Linked Mode. Then I got bitten. It turned out that if you do a vCenter 5.1 simple install, the Single Sign On component gets installed like a single site SSO. So I had to reinstall, and that's what I did.

Start the installation like you usually would, until you can select the “Create the primary node for a new vCenter Single Sign On installation”:

Then select “Create the primary node for a new vCenter Single Sign On installation”:

And normally continue the wizard.

At the secondary site you install a secondary SSO node pointing to the primary site. Just start the installation and on the Deployment Type panel select “Join an existing vCenter Single Sign On installation”:

Set the node type to Multisite, and point to the Single Sign On primary node that you created before. Also enter the FQDN, Single Sign On HTTPS port (7444) by default, and the password for admin@System-Domain for the primary Single Sign On node:

Also register your administrator group with SSO:

Now finish the wizard.

You can now use this SSO setup for vCenter Linked Mode.