Manual

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Chapter 31 VRRP Configuration
31.1 Introduction to VRRP
VRRP (Virtual Router Redundancy Protocol) is a fault tolerant protocol designed to enhance
connection reliability between routers (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 normal and uninterrupted communication can
be achieved.