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 (Virtual LAN). 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.
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 (Extreme Standby Router Protocol), as described in Using ELRP with ESRP. Protocols such as EAPS (Extreme Automatic Protection Switching) 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.
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