Multicast and Routing Guide K/KA/KB.15.15

receive any VRRP packets and the master down time expires, the owner router can take control of
its VIP immediately.
If the value of the master down time (3 * advertisement interval) is greater than the
preempt delay time, the owner router will wait until the PDT expires before taking
control of its VIP.
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.
Possible configuration scenarios
PDT=zero seconds
This is the default behavior. It works in the same way that VRRP works currently.
PDT is greater than or equal to the master down time (3 times the advertisement interval)
a. An owner VR after reboot—waits for the master down time. If the owner router does not receive
a packet during this time, it becomes the master. If it receives a VRRP advertisement from its
peer during this time, it waits until the expiration of the preempt delay time before becoming
the master.
b. A backup VR after reboot—waits for the master down time. If the backup router does not
receive a packet during this time, it becomes the master. If it receives a VRRP advertisement
from its peer during this time, and it has a higher priority value than this peer, it waits until
the expiration of the preempt delay time before becoming the backup.
PDT is less than the master down time
a. Owner router—becomes the master after expiration of the PDT.
b. Backup router—becomes the backup after expiration of the PDT if it does not receive a VRRP
advertisement from a higher priority peer (or the owner.)
When the PDT is not applicable
Once the router has rebooted and is in steady state VRRP operation, the PDT is not applicable if:
• The VRRP VLAN goes down and comes back up.
• The VR is disabled and re-enabled.
• VRRP is globally disabled and then re-enabled.
286 Virtual Router Redundancy Protocol (VRRP)