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)
1.
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.
2.
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
1.
Owner router—becomes the master after expiration of the PDT.
2.
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.
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Aruba 3810 / 5400R Multicast and Routing Guide for ArubaOS-
Switch 16.08
