i had to move hard disks containing installed esxi 5.1 from one server to another. after i booted up a new server there was an unexpected surprise for me… vmware ‘kept’ the mac address assigned to the management interface from the old machine. so for a while i had duplicate mac address: legitimately on the old machine and spoofed by vmware on the new one which resulted in packet losses.
vmware kb1031111 explains the situation. to force esxi to use a new mac i had to run:
esxcfg-advcfg -s 1 /Net/FollowHardwareMac
and reboot the server