HP 3500YL - Manual (Page 363)

Virtual Router Redundancy Protocol (VRRP)
Using the Pre-empt Delay Timer
During this waiting period, if the Owner router receives a VRRP packet for its
virtual IP address from the Backup router, it will wait until the PDT expires
before taking control of its virtual IP address. If the Owner router does not
receive any VRRP packets and the Master down time expires, the Owner
router can take control of its virtual IP address immediately.
If the value of the Master down time (3 * advertisement interval) is greater than
the preempt delay time, then the Owner Router will wait until the PDT expires
before taking control of its virtual IP address.
Selecting a Value for the PDT
You should select the value for the PDT carefully to allow time for OSPF to
populate the Owner router’s route tables. The choice depends on the
following:
■ The OFPF router dead interval—the number of seconds the OSPF
router waits to receive a hello packet before assuming its neighbor is
down.
■ The number of router interfaces that participate in OSPF
■ The time it may take from reception of the OSPF packets to when the
population of the route table is completed.
There are trade-offs between selecting a small advertisement value and a large
preempt delay time. A small advertisement value results in a faster failover to
the Backup router. A larger PDT value allows OSPF to converge before the
Owner router takes back control of its virtual IP address.
Choosing a large PDT value (greater than the Master down time) may result
in an unnecessary failover to the Backup router when the VRRP routers
(Owner and Backup) start up together. Choosing a large advertisement
interval and thereby a large Master down time results in a slower failover to
the Backup router when the Owner router fails.
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