The Catalyst 6500 Switch that runs Cisco IOS software uses Maintenance Operations Protocol (MOP) packets in order to advertize its existence to Digital Equipment Corporation machines that use the MOP protocol. A switch periodically broadcasts MOP packets in order to identify itself as a MOP host. As a result, MOP packets are counted, even when DECnet is not actively used.
The MOP is a Digital Equipment Corporation protocol that provides a way to perform primitive maintenance operations on DECnet systems. For example, MOP can be used to download a system image to a diskless station.