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DES-7200 Configuration Guide Chapter 1 VRRP Configuration
1-1
1 VRRP Configuration
1.1 Overview
The Virtual Router Redundancy Protocol (VRRP) is designed to work in the
active/standby mode to ensure that the standby router can take over the work without
affecting internal and external data communication and modifying the parameters of
internal networks when the master router fails. Multiple devices within a VRRP group
are mapped to a virtual device. The VRRP ensures one and only one device to send
packets on behalf of the virtual device at one time, while the host sends messages to
that virtual device. The device that forwards packets is elected as the master device. If
that device cannot work due to some reason, the one in standby status will be selected
to replace it and become the active device. With VRRP, the hosts in the LAN seem to
use only one router. The route connectivity is also guaranteed even when the
currently-used first-hop router fails.
RFC 2338 defines the IP packet format in VRRP type and its working mechanism. The
VRRP message means a kind of multicast message with specified destination address,
which is sent by the master router by schedule to indicate its operation and are also
used to elect the master router. The VRRP allows another router to automatically take
over the operations when the router that undertaking route forwarding function in the IP
LAN fails, thus implementing the hot-backup and error-tolerance of IP routing and
ensuring the continuity and reliability of host communication in the LAN. Redundancy
is implemented for a VRRP application group through multiple devices, but only one
device acts as the master device at any time to undertake the route forwarding function.
The others are in the backup roles. Inter-router switching in the VRRP application
group is fully transparent for the host in the LAN. The RFC 2338 defines the router
switching rules:
The VRRP protocol adopts the preemption method to select the master router.
First, it compares the VRRP priorities that are set for the interfaces of the routers
a VRRP group. The one with the highest priority becomes the master router and
its status will become Master. If the priority of the routers is identical, compare the
master IP addresses of the network interfaces, the one with larger IP address will
become the master router to forward packets.
After the master device is elected, the others are in the standby status and
monitor the status of the master device through the VRRP message sent by the
master device. In normal operation, the master router sends a VRRP message at
an interval, called advertised message, to notify the standby devices. The master