Using ELRP to Perform Loop Tests

The Extreme Loop Recovery Protocol (ELRP) is used to detect network loops in a Layer 2 network. A switch running ELRP transmits multicast packets with a special MAC destination address out of some or all of the ports belonging to a VLAN. All of the other switches in the network treat this packet as a regular, multicast packet and flood it to all of the ports belonging to the VLAN. When the packets transmitted by a switch are received back by that switch, this indicates a loop in the Layer 2 network.



The ELRP source MAC address is changed from "00:e0:2b:00:00:01" to “0e:Switch-MAC”.

After a loop is detected through ELRP, different actions can be taken such as blocking certain ports to prevent loop or logging a message to system log. The action taken is largely dependent on the protocol using ELRP to detect loops in the network.

You can use ELRP on a “standalone” basis or with other protocols such as ESRP, as described in Using ELRP with ESRP. Protocols such as EAPS require that a network have a ring topology to operate. In this case you can use ELRP to ensure that the network has a ring topology.

The “standalone” ELRP commands determine whether or not a network has an Layer 2 loop.