By default Cisco routers and switches periodically test their (Fast) Ethernet links by sending out Loopback frames (ethertype 0x9000) 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.

In the exchange environment, this is not useful since we use MAC timeouts that are larger than the typical BGP and/or ARP timeouts. In fact, the keepalives may actually cause port security violations if they are being sent by an intermediate switch.