Using ELRP with ESRP to Recover Loops

ELRP sends loop-detect packets to notify ESRP about loops in the network.

In an ESRP environment, when the current master goes down, one of the slaves becomes the master and continues to forward Layer 2 and Layer 3 traffic for the ESRP domain. If a situation occurs when a slave incorrectly concludes that the master is down, the slave incorrectly assumes the role of master. This introduces more than one master on the ESRP domain which causes temporary loops and disruption in the network.

ELRP on an ESRP Pre-Master Switch

A pre-master switch is an ESRP switch that is ready to transition to master but is going through possible loop detection.

A pre-master periodically sends out ELRP loop-detect packets (ELRP PDUs) for a specified number of times and waits to make sure that none of the sent ELRP PDUs are received. Transition to master occurs only after this additional check is completed. If any of the ELRP PDUs are received, the switch transitions from pre-master to slave state. You configure pre-master ELRP loop detection on a per ESRP domain basis.

ELRP on an ESRP Master Switch

A master switch is an ESRP switch that sends ELRP PDUs on its ESRP domain ports.

If the master switch receives an ELRP PDU that it sent, the master transitions to the slave. While in the slave state, the switch transitions to the pre-master rate and periodically checks for loops before transitioning to the master. The pre-master process is described in ELRP on an ESRP Pre-Master Switch. You configure the master ELRP loop detection on a per ESRP domain basis.