Multicast and Routing Guide K/KA/KB.15.15

Pinging the virtual IP of a backup router
When in compliance with RFC 3768 , only owner VRs reply to ping requests (ICMP echo requests)
to the VIP. When the virtual IP ping option is enabled, a backup VR operating as the master can
respond to ping requests made to the VIP. This makes it possible to test the availability of the default
gateway with ping. A non-owner VR that is not master drops all packets to the VIP.
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 HP devices
There can be an issue with VRRP preempt mode if an older HP 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 HP devices, whereas it is disabled by default
on later models such as the HP Series 5400zl. HP recommends 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
Pinging the virtual IP of a backup router 285