Master Down Interval; Skew Time; Preempt Mode - Alcatel-Lucent 7705 Configuration Manual

VRRP Components

Master Down Interval

The master down interval is the time that the master router can be down before backup takes
over. The master down interval is used to specify the master down timer. If the master down
timer expires, the backup virtual router enters the master state. To calculate the master down
interval, the virtual router uses the following formula:
Master Down Interval = (3 × Operational Message Interval) + Skew Time
The operational message interval is dependent upon the state of the inherit parameter. If the
inherit parameter is enabled, the operational message interval is derived from the current
master's message interval field in the VRRP advertisement message. If the inherit parameter
is disabled, the operational message interval must be equal to the locally configured message
interval.
The master down timer is only operational if the local virtual router is operating in backup
mode.

Skew Time

The skew time is used to add a time period to the master down interval. Skew time is not a
configurable parameter. It is derived from the current local priority of the virtual router. To
calculate the skew time, the virtual router uses the following formula:
For IPv4: skew time = ((256 – priority) / 256) s
The higher the priority value is, the smaller the skew time. This means that virtual routers
with a lower priority transition to master slower than virtual routers with higher priorities.

Preempt Mode

Preempt mode is a configured true or false value that controls whether a virtual router backup
with a higher priority preempts a lower-priority master. Preempt mode cannot be set to false
on the owner virtual router.The IP address owner always becomes master if available. The
default value for preempt mode is true.
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Note: A message interval is the same as an advertisement interval.
7705 SAR OS Router Configuration Guide