AT-S63 Management Software Features Guide
Section VII: Routing 407
Backup Switches
All the other switches participating in the virtual router are designated as
backup switches. A switch can be part of several different virtual routers
on one LAN, provided that all the virtual routers have different virtual router
identifiers.
When a switch functions as a backup for a virtual router, it does the
following:
Receives advertisement packets from the master switch and checks
that the information contained in them is consistent with their own
configuration, ignoring and discarding advertisement packets that do
not match.
Assumes the role of master switch for the virtual router if an
advertisement packet is not received for a given period of time (the
“master-down” period), based on the specified advertisement interval.
The “master-down” time is approximately three times the
advertisement interval.
Assumes the role of master switch if it receives an advertisement
packet from another switch with a lower priority than its own, if preempt
mode is on.
When the master switch fails, the backup switch assumes control and
starts processing traffic.
If a backup switch is about to assume the role of master of the virtual
router because it has not received an advertisement for the “master-down”
period, it first checks the operational status of the interface to which the
virtual router is attached. If the interface is down, it does not enter the
master state. Instead, it says in the backup state and checks the interface
again after another “master-down” period, assuming that it does not
receive an advertisement during that time.