Product specifications

Chapter 6: Connection Management Efficient Networks
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Router family
Command Line Interface Guide
Page 6-16 Efficient Networks
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VRRP Backup
When a router is defined as a static default gateway and no other dynamic routing
protocol or router discovery protocol is used (such as RIP), the gateway becomes a
critical link in the network. If that router fails, that critical link would be broken. It,
therefore, may be appropriate to set up other routers as backups that can serve as
the static default gateway if necessary.
The Virtual Router Redundancy Protocol (VRRP), as defined in RFC 2338, allows
other IP routers in a LAN to provide immediate and automatic backup to a failed IP
router. VRRP is a protocol that defines how backup routers monitor the status of a
master router and take over its function if it fails. The new master router adopts the IP
and MAC address of the original master, so that the hosts configured with the single
default gateway maintain their network connection.
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 master router and its backups;
other values must differ (as discussed in the following sections).
VRRP configuration requires these basic steps:
1. Define logical interfaces.
2. Define the ID of the Virtual Router (VRID).
3. Define the VRRP attributes of the Virtual Router.
4. Save the changes and either restart the VRRP interface or reboot the 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 “IP Subnets” on page 6-1.)
The VRRP interface is for VRRP use 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.