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Normally in OTV, unknown unicast frames are not flooded between OTV sites and MAC addresses are not learned across the overlay interface.  Any unknown unicast messages that reach the OTV edge device are blocked to prevent layer 2 errors spreading to remote sites.  It is assumed the end points are not silent or unidirectional.  If there is any silent hosts or Microsoft unicast mode NLB is used in your data center, your host will be "disappeared" from the view of other OTV sites.  In 6.2(2) and later, a new feature called selective unicast flooding is introduced and you can issue the command on the OTV VDCs at the site where the server / silent host exists, in result the specified destination MAC address is flooded to all other edge devices in the OTV overlay network with that unknown unicast traffic.