I am going to touch a specific issue that i observed for XenServer 6.2. The pool has bonded management interface. If a new server is tried to introduce, the management interface for the new server goes down and doesn't come up.
when looking into the networking tab, you would see one interface with unknown tag line and another with management interface without any IP address.
Say NIC 0 and NIC 1 has been bonded
Try the specific command sets in order :-
ifconfig eth0 down
ifconfig eth1 down
ifconfig eth0 up
ifconfig eth 1 up
and voila the XenServer comes up with out complaining any further.