[CentOS] the wandering MAC?

Hi all,

I have what appears to be a truly puzzling problem. I've got this P4 32-bit machine running CentOS 5.5 with XEN that has two NICs: one onboard, an Intel Corporation 82540EM Gigabit and one on an expansion card, Realtek Semiconductor Co., Ltd. RTL-8169 Gigabit. The second one is recognized as eth1.

What's happening is, it is showing up under one of the two MAC's: either 00:0a:cd:1a:c1:71 or 00:00:00:00:c1:71. If you reboot it the MAC stays the same; if you shutdown and do a full powerdown it seems to change.

Obviously, after it goes from one MAC to another you have to play with the start-up scripts for this interface to start up correctly and this becomes a major annoyance.

Any idea what all of this mess could mean?

Thanks.

Boris.