User`s guide
88 XSR User’s Guide
IP Routing Protocols Chapter 5
Configuring IP
How the VRRP Works
Multiple IP routers on a single broadcast LAN comprise a single virtual
router, which has a unique virtual IP address and virtual MAC address. Hosts
on the LAN configure the VR as their default router (default gateway).
Devices that provide support for a VR form a VRRP group. The device acting
as the VR is designated the master of the group.
At any one time, only one of the routers acts as the VR, forwarding packets
from hosts on the LAN. If that router goes down, the VRRP provides a
method by which one of the other routers in the group can take over the
virtual IP address and MAC address in a timely manner.
When the VRRP is started, the IP router sends and receives VRRP
advertisements until a master is chosen. If the IP router does not become the
master, it continues to listen to advertisements from the master of the group.
If the IP router becomes the master of the group, it begins sending VRRP
advertisements and adds VRRP group information to the interface set. Once
added, any Ethernet frame for the virtual MAC address is received by the IP
router. Any ARP requests for the virtual IP address are responded to using
the virtual MAC address.
If the IP router ceases to be the group master, it removes the VRRP group
information from the system and continues to listen for VRRP advertisements
from the new master.
Different States of a VRRP Router
Underlying how VRRP operates are three states the VRRP router experiences:
initialize, backup, and master. Initialize is the first state and involves the
following steps:
A VRRP router checks the virtual IP address to learn if it is the master.
If it owns that address, it realizes it is the master and its priority is 255.
If the priority equals 255, the VRRP router advertises itself as the
master, broadcasts an ARP message to all IP addresses associated
with the VR’s IP address, starts the advertisement timer and
transitions to the master state.
If priority is less than 255, the VRRP router transitions to backup state.