NOTE:
This feature is not a part of RFC 3768. Enabling this feature results in non-compliance with RFC
3768 rules.
Using the Pre-empt Delay Timer (PDT)
To maintain availability of the default gateway router, the VRRP advertises a "virtual" router to the hosts. At least
two other physical routers are configured to be virtual routers, but only one router provides the default router
functionality at any given time. If the owner router or its VLAN goes down, the backup router takes over. When the
owner router comes back on line (fail-back), it takes control of the VIP that has been assigned to it. It begins
sending out VRRP advertisement packets at regular intervals. The backup router receives the VRRP
advertisement packet and transitions to the backup state.
When OSPF is also enabled on the VRRP routers
When OSPF is enabled on the routers and a fail-back event occurs, the owner router immediately takes control of
the VIP and provides the default gateway functionality. If OSPF has not converged, the route table in the owner
router may not be completely populated. When the hosts send packets to the default gateway, the owner router
may not know where to send them and packets may be dropped.
CAUTION:
While you can run OSPF and VRRP concurrently on a router, it is best not to run VRRP with other
routing protocols, such as RIP or OSPF, on the same interface or VLAN, as this can create
operational issues.
Configuring the PDT
The VRRP PDT allows you to configure a period of time before the VR takes control of the VIP. It does not
transition to the master state until the timer period expires. The timer value configured should be long enough to
allow OSPF convergence following OSPF updates.
The PDT is applied only during initialization of the router, that is, when the router is rebooting with the VRRP
parameters present in the startup config file.
VRRP preempt mode with LACP and older devices
There can be an issue with VRRP preempt mode if an older device is the intermediate device connecting to a
VRRP router and has LACP set in "enable, passive" mode. This mode is set by default on older devices, whereas
it is disabled by default on later models. It is recommended that you use compatible LACP settings on devices
that connect with VRRP routers on VRRP VLANs.
What occurs at startup
When the owner router comes online, it waits for the configured amount of time before taking control of the VIP.
This period of time is calculated as follows:
If the value of the master down time (3 * advertisement interval) is less than or equal to the preempt delay
time, the owner router will wait until the master down time (3 * advertisement interval) has expired.
During this waiting period, if the owner router receives a VRRP packet for its VIP from the backup router, it waits
until the PDT expires before taking control of its VIP. If the owner router does not 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.
Chapter 16 Virtual Router Redundancy Protocol (VRRP)
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