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 PDT. 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 VIP.

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.