Perhaps I am using this feature wrong, but I don't think so. This morning, I am trying to get mac-address accounting working on a 7507MX/RSP8/12.0.17S/GEIP+ running ISL, to indentify large peers off an ethernet peering fabric. I've done 'ip accounting mac-address out'. I then wait. Then, I: core1.nyc#sho int g0/0/0 mac-accounting GigabitEthernet0/0/0 to external peers and customers Output (475 free) 0001.64da.bc00(3 ): 2 packets, 179 bytes, last: 3036ms ago 0004.de2b.e41c(9 ): 1 packets, 99 bytes, last: 3036ms ago 00e0.52a6.1f00(11 ): 1 packets, 99 bytes, last: 3036ms ago 0100.0c00.0000(13 ): 57198 packets, 37155973 bytes, last: 388ms ago 0030.7bee.fc54(13 ): 2 packets, 218 bytes, last: 2836ms ago All of the mac-addresses listed in this 'sho' are valid, except for '0100.0c00.0000', which doesn't exist: core1.nyc#sho arp | inc 0100 core1.nyc# All the others are valid, yet they are way, and I mean *way* under the amounts that I know I am sending to that peer. Any clues? I've searched CCO/Bug Toolkit, I don't see anything relevant.