HP 3500YL - Manual (Page 361)

Virtual Router Redundancy Protocol (VRRP)
Using the Pre-empt Delay Timer
Using the Pre-empt Delay Timer
Overview
In order to maintain availability of the default gateway router, the Virtual
Router Redundancy Protocol (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 virtual
IP address that has been assigned to it. It begins sending out VRRP advertise­
ment 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 virtual IP address 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.
C a u t i o n 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 Pre-empt Delay Timer
The VRRP Pre-empt Delay Timer (PDT) allows you to configure a period of
time before the VR takes control of the virtual IP address. 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.
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