Chapter 17 Vrrp Configuration; Introduction To Vrrp - Accton Technology ES4626 Management Manual

Chapter 17 VRRP Configuration

17.1 Introduction to VRRP

VRRP (Virtual Router Redundancy Protocol) is a redundancy protocol. It uses a
backup mechanism to increase reliability of the router (or the layer 3 switch) to connect
the outside network. It is designed for the local area network which supports multicast or
broadcast, such as Ethernet. It is proposed by IETF, and widely used these days.
Normally, the default gateway should be configured on all the hosts in the LAN. When
the hosts send packets whose destinations are not in the same subnet, these packets are
sent to the default gateway. This configuration ensures the connection between the hosts
in the subnet and the outside network. But when the connection between the default
gateway and the outside network is down, all the hosts in the subnet can't communicate
with the outside network.
The VRRP is developed to solve this problem. The VRRP is run on the multiple
routers in the LAN. These routers form a virtual router and are called a standby group. In
the standby group, there are one active router (called Master) and one or several backup
routers (called Backup). The master router is responsible for forwarding the packets,
whereas the backup routers serve as backups for the master router.
The virtual router has its virtual IP address which can be the same as the IP address
of an interface of a router in the standby group. The backup routers also have their IP
addresses. All the hosts in the LAN only need to set their default gateway to the virtual IP
address of the virtual router, then they can communicate with the outside network. In fact,
only the master router forwards the traffic. When the master router is down, one backup
router takes it over, and the communication with the outside network is maintained.
Let's make a sum-up; In the VRRP standby group, there is always a master router
which forwards the traffic; the other routers serve as backup routers. They monitor the
status of the master router. When the master router is down, the backup routers select a
new master router which forwards the traffic. This new election takes a very short time, so
the hosts in the LAN can communicate with the outside work through the virtual router.
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