A layer 2 Ethernet frame is sent with a source and destination MAC address. Given a 100Base-TX (copper) trunk between two Cisco switches, I can do a "show interface fa 0/0" on S1 to see the MAC address assigned to the trunking interface, then go to Switch2 and do a "show mac address-table" and find the MAC address of the S1 fa 0/0 interface as a dynamically learned MAC in the table. Given a similar setup with a 1000Base-X (fiber GBIC) trunk, the MAC address shown in "show interface gi 0/0" on S1 does not show up in the MAC address-table of S2. Everything I can find online indicates that 1000Base-X uses largely the same layer 2 format as copper connections. There's some slight alterations - minimum frame size is slightly larger - but the fundamentals of the frame structure appear to be the same, including transmission with a source and destination L2 address. Why doesn't the address of gi 0/0 show up in the MAC address table of the connection switch? The only thing which seems to make sense would be that the GBIC has its own MAC address, almost as if its acting as a mini 2-port switch or hub, with the switch-assigned MAC address showing up on the interface connection and a different MAC address assigned to the fiber side. If this is the case, is there any way to see the GBIC MAC address on the switch? (I've tried to look up the details in IEEE 802.3z but it doesn't seem to be available without an IEEE membership or purchasing the standard. I find the base 802.3 PDFs for download, but not 802.3z.)