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By default Cisco routers and switches periodically test their (Fast) Ethernet links by sending out Loopback frames (ethertype 0×9000) addressed to themselves. Call it a “L2 self-ping” if you will. In a switched environment it can be used to test the functionality of the switch and/or keep the router’s MAC address in the switch’s address table. Another thing what this Loopback frames do, is to check for a loop. If there is a loop in the network, the resulting Loopback frame will be seen by the sending switch and the port will be err-disabled.

The folllowing picture shows a Loopback frame; As we can see, the SA=DA.

Recently I had a problem in a network which was caused by the loopback mechanism; when we tried to add a switch to the VTP domain with redundant connections, I noticed that this loopback mechanism had err-disabled all the uplink ports of a lot of edge switches! I couldn’t take my edgeswitches anymore and I had to manually take these ports out of the err-disbled state. The following syslog messages were generated on the switches;

ETHCNTR-3-LOOP_BACK_DETECTED: Loop-back detected on gig0/2

We activated the err-disabled recovery mechanism to eliminate this problem. You can also disable the keepalive on uplink ports with the no keepalive interface command for the uplink ports.

Cisco has documented this problem as bug ID CSCea46385. Cisco say’s that starting in 12.2SE based releases, keepalives are NO longer sent by default on fiber and uplink interfaces.