Specifications

108 Chapter 4. Configuring Special Features
The following illustration shows two routers connecting a LAN to the Internet. By using VRRP, the backup router
can take over as the gateway if the master router fails.
Routers using VRRP send out advertisement packets at intervals to let the other VRRP routers on the LAN know
that they are still up. The other VRRP routers realize that a router is down when no advertisement packets have
been received for the minimum down interval. The VRRP router assigned the highest priority takes over for the
failed router. When the failed router is restored, it can automatically preempt the backup router and resume its
function in the network.
VRRP Configuration
To configure a LAN to use VRRP, you must enter configuration commands into every router that is to be
provided with backup or that is to serve as backup to another router. Certain values must be the same between the
original router and its backups; other values must differ (as discussed in the following sections).
VRRP configuration requires three basic steps:
1. Define logical interfaces.
2. Define the ID of the Virtual Router (VRID).
3. Define the VRRP attributes of the Virtual Router.
Defining the VRRP Interface
Each router that is to use VRRP must have at least two logical Ethernet interfaces defined, one to be used as the
VRRP interface and the other as the management interface. (Logical interfaces are discussed under Multiple IP
Subnets, page 77.)
The VRRP interface is for VRRP only; it cannot be used for any other purpose. Unlike other logical interfaces, the
VRRP interface does not use the usual Ethernet MAC address associated with the router. Instead, it uses the
VRRP MAC address as defined in RFC 2338, that is, 00005e0001xx where xx is the VRID.
IP Address
Every logical interface is assigned its own IP address, or range of addresses, that is unique on the LAN. The
VRRP interface must be assigned the IP address that serves as the default static gateway for other devices on the
LAN.
Master
Backup
Ethernet LAN
Internet
VRRP Routers