Ghost MAC addresses

I'm seeing some odd ARP broadcasts on my network:

11:22:28.215534 00:0c:76:4e:04:c8 > Broadcast, ethertype ARP (0x0806),
length 60: arp who-has 172.16.100.103 (Broadcast) tell 172.16.100.103

(Why would 172.16.100.103 be asking other machines who has
172.16.100.103 and then tell 172.16.100.103 ??? Doesn't make a lot of
sense :-) )

I'm trying to use sh mac-address-table on my 2970s to track down where
this address is, but...

2970_1#sh mac-address-table address 000c.764e.04c8
Mac Address Table
-------------------------------------------

Vlan Mac Address Type Ports
---- ----------- -------- -----
3 000c.764e.04c8 DYNAMIC Gi0/4
Total Mac Addresses for this criterion: 1
2970_1#sh mac-address-table address 000c.764e.04c8
Mac Address Table
-------------------------------------------

Vlan Mac Address Type Ports
---- ----------- -------- -----
3 000c.764e.04c8 DYNAMIC Gi0/24
Total Mac Addresses for this criterion: 1
2970_1#sh mac-address-table address 000c.764e.04c8
Mac Address Table
-------------------------------------------

Vlan Mac Address Type Ports
---- ----------- -------- -----
3 000c.764e.04c8 DYNAMIC Gi0/4
Total Mac Addresses for this criterion: 1
2970_1#sh mac-address-table address 000c.764e.04c8
Mac Address Table
-------------------------------------------

Vlan Mac Address Type Ports
---- ----------- -------- -----
3 000c.764e.04c8 DYNAMIC Gi0/4
Total Mac Addresses for this criterion: 1
2970_1#sh mac-address-table address 000c.764e.04c8
Mac Address Table
-------------------------------------------

Vlan Mac Address Type Ports
---- ----------- -------- -----
3 000c.764e.04c8 DYNAMIC Gi0/24
Total Mac Addresses for this criterion: 1


Gi0/24 is a trunk to 2970_2 and Gi0/4 is a trunk to 2970_3

Why the fsck is this address dancing around my network? 000c.764e.04c8
is MSI, and I don't have no stinkin' MSI crap in my cage :-) There's a
PIX firewall between me and the world, and the only possibility of
"extra" traffic getting in might be through the crossconnects between
the two cabinets, but that's all they are... crossconnects. I thought
maybe they went to a switch or something, but no... they're physical
cables.

--
* John Oliver http://www.john-oliver.net/ *
.