Accton Technology ES4626 - Manual (Page 810)

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Chapter 20 VRRP Configuration
20.1 Introduction to VRRP
VRRP (Virtual Router Redundancy Protocol) is a fault tolerant protocol designed to
enhance connection reliability between routes (or L3 Ethernet switches) and external
devices. It is developed by the IETF for local area networks (LAN) with
multicast/broadcast capability (Ethernet is a Configuration Example) and has wide
applications.
All hosts in one LAN generally have a default route configured to specified default
gateway, any packet destined to an address outside the native segment will be sent to
the default gateway via this default route. These hosts in the LAN can communicate with
the external networks. However, if the communication link connecting the router serving
as default game and external networks fails, all hosts using that gateway as the default
next hop route will be unable to communicate with the external networks.
VRRP emerged to resolve such problem. VRRP runs on multiple routers in a LAN,
simulating a "virtual" router (also referred to as a "Standby cluster") with the multiple
routes. There is an active router (the "Master") and one or more backup routers (the
"Backup") in the Standby cluster. The workload of the virtual router is actually undertaken
by the active router, while the Backup routers serve as backups for the active router.
The virtual router has its own "virtual" IP address (can be identical with the IP
address of some router in the Standby cluster), and routers in the Standby cluster also
have their own IP address. Since VRRP runs on routes or Ethernet Switches only, the
Standby cluster is transparent to the hosts with the segment. To them, there exists only
the IP address of the Virtual Router instead of the actual IP addresses of the Master and
Backup(s). And the default gateway setting of all the hosts uses the IP address of the
Virtual Router. Therefore, hosts within the LAN communicate with the other networks via
this Virtual Router. But basically, they are communicating with the other networks via the
Master. In the case when the Master of the Standby cluster fails, a backup will take over
its task and become the Master to serve all the hosts in the LAN, so that uninterrupted
communication between LAN hosts and external networks can be achieved.
To sum it up, in a VRRP Standby cluster, there is always a router/Ethernet serving
as the active router (Master), while the rest of the Standby cluster servers act as the
backup router(s) (Backup, can be multiple) and monitor the activity of Master all the time.
Should the Master fail, a new Master will be elected by all the Backups to take over the
work and continue serving the hosts within the segment. Since the election and take-over
duration is brief and smooth, hosts within the segment can use the Virtual Router as