Instruction Manual

Virtual Router Redundancy Protocol (VRRP)

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.

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 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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