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 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.
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![[CAUTION: ]](images/caution.gif) |
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. |
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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.
There can be an issue with VRRP preempt mode if an older HPE device (2524, 2650, 2848, 3400, or 5300) 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 such as the Series 5400zl. Hewlett Packard Enterprise recommends that you use compatible LACP settings on devices that connect with VRRP routers on VRRP VLANs.
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.
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:
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The OFPF router dead interval—the number of seconds the OSPF router waits to receive a hello packet before assuming its neighbor is down.
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The number of router interfaces that participate in OSPF
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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.
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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.
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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.