Alcatel-Lucent 7750 SR-OS, Configuration Manual

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Page 316 7750 SR OS Router Configuration Guide
VRRP Priority Control Policy Priority Events
The main function of a VRRP priority control policy is to define conditions or events that impact
the system’s ability to communicate with outside hosts or portions of the network. When one or
multiple of these events are true, the base priority on the virtual router instance is either
overwritten with an explicit value, or a sum of delta priorities is subtracted from the base priority.
The result is the in-use priority for the virtual router instance. Any priority event may be
configured as an explicit event or a delta event.
Explicit events override all delta events. When multiple explicit events occur, the event with the
lowest priority value is assigned to the in-use priority. As events clear, the in-use priority is
reevaluated accordingly and adjusted dynamically.
Delta priority events also have priority values. When no explicit events have occurred within the
policy, the sum of the occurring delta events priorities is subtracted from the base priority of each
virtual router instance. If the result is lower than the delta in-use priority limit, the delta in-use
priority limit is used as the in-use priority for the virtual router instance. Otherwise, the in-use
priority is set to the base priority less the sum of the delta events.
Each event generates a VRRP priority event message indicating the policy-id, the event type, the
priority type (delta or explicit) and the event priority value. Another log message is generated
when the event is no longer true, indicating that it has been cleared.
Priority Event Hold-Set Timers
Hold-set timers are used to dampen the effect of a flapping event. A flapping event is where the
event continually transitions between clear and set. The hold-set value is loaded into a hold set
timer that prevents a set event from transitioning to the cleared state until it expires.
Each time an event transitions between cleared and set, the timer is loaded and begins to count
down to zero. If the timer reaches zero, the event will be allowed to enter the cleared state once
more. Entering the cleared state is always dependent on the object controlling the event
conforming to the requirements defined in the event itself. It is possible, on some event types, to
have a further set action reload the hold set timer. This extends the amount of time that must expire
before entering the cleared state.
For an example of a hold-set timer setting, refer to LAG Degrade Priority Event on page 317 .