Extreme Standby Router Protocol
ExtremeWare XOS 11.3 Concepts Guide
476
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 prior to transitioning to
the master. The pre-master process is described in “ELRP on an ESRP Pre-Master Switch” on page 477.
You configure the master ELRP loop detection on a per ESRP domain basis.
Configuring ELRP
This section describes the commands used to configure ELRP for use with ESRP. By default, ELRP is
disabled.
Configuring Pre-Master Polling
If you enable the use of ELRP by ESRP in the pre-master state, ESRP requests ELRP packets sent to
ensure that there is no loop in the network prior to changing to the master state. If no packets are
received, there is no loop in the network. By default, the use of ELRP by ESRP in the pre-master state is
disabled.
To enable the use of ELRP by ESRP in the pre-master state on a per-ESRP domain basis, and to
configure how often and how many ELRP PDUs are sent in the pre-master state, use the following
command:
configure esrp <esrpDomain> elrp-premaster-poll enable {count <count> | interval
<interval>}
Where the following is true:
● esrpDomain—Specifies an ESRP domain name.
● count—Specifies the number of times the switch sends ELRP PDUs. The default is 3, and the range
is 1 to 32.
● interval—Specifies how often, in seconds, the ELRP PDUs are sent. The default is 1 seconds, and
the range is 1 to 32 seconds.
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