Strange L2 failure

Jack Bates jbates at
Sat Jan 29 15:24:54 CST 2011

Has anyone seen issues with IOS where certain MACs fail?

54:52:00 (kvm) fails out an old 10mbit port on a 7206 running 12.2 SRE. 
I've never seen anything like this. DHCP worked, ARP worked, and arp 
debugging showed responses for arp to the MAC, however, tcpdump on the 
host system showed no unicast or arp responses coming from the router, 
while the switch management ip and other stuff on the local segment 
communicated fine with the vm. This broke for IPv6 as well.

I changed the vm's MAC to 54:51:00, 50:52:00 and still failed. Changed 
it to 40:52:00 and it works for both v4 and v6. Was there a change which 
would cause certain hardware to not accept a MAC starting with 50: or 


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