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7750 SR OS Router Configuration Guide Page 177
Master Down Interval
The master down interval is a calculated value used to load the master down timer. When the
master down timer expires, the virtual router enters the master state. To calculate the master down
interval, the virtual router evaluates the following formula:
Master Down Interval = ((3 x Operational Advertisement Interval) + Skew Time) seconds)
The operational advertisement interval is dependent upon the state of the inherit parameter. When
the inherit parameter is enabled, the operational advertisement interval is derived from the current
master’s advertisement interval field in the VRRP advertisement message. When inherit is
disabled, the operational advertisement interval must be equal to the locally configured
advertisement interval.
The master down timer is only operational when the local virtual router is operating in backup
mode.
Preempt Mode
Preempt mode is a true or false configured value which controls whether a specific backup virtual
router preempts a lower priority master. The IP address owner will always become master when
available. Preempt mode cannot be set to false on the owner virtual router. The default value for
preempt mode is true.
When preempt mode is true, the advertised priority from the incoming VRRP advertisement
message from the current master is compared to the local configured priority. If the local priority is
higher, the received VRRP advertisement message is discarded. This will result in the eventual
expiration of the master down timer causing a transition to the master state. If the received priority
is equal to the local priority, the message is not discarded and the current master will not be
discarded. Note that when in the backup state, the received primary IP address is not part of the
decision to preempt and is not used as a tie breaker when the received and local priorities are equal.
When
preempt is enabled, the virtual router instance overrides any non-owner master with an in-
use message priority value less than the virtual router instance in-use priority value. If
preempt is
disabled, the virtual router only becomes master if the master down timer expires before a VRRP
advertisement message is received from another virtual router.