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When you designing vSphere 5.1 you have to implement vCenter SSO. Therefore you have to make design decision what SSO mode to choose.
There are actually three available options
HA (don't mix with vSphere HA)
Justin King wrote excellent blog post about SSO here and it is worth source of information to make right design decision. I fully agree with Justin and recommending Basic SSO to my customers if possible. SSO Server protection can be achieved by standard backup/restore methods and SSO High Availability can be increased by vSphere HA. All these methods are well known and long time used.
You have to use Multisite SSO when vCenter linked-mode is required but think twice if you really need it and benefits overweight drawbacks.