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Understanding VRRP

For Ethernet, Fast Ethernet, Gigabit Ethernet, 10-Gigabit Ethernet, and logical interfaces, you can configure the Virtual Router Redundancy Protocol (VRRP) or VRRP for IPv6. VRRP enables hosts on a LAN to make use of redundant routing platforms on that LAN without requiring more than the static configuration of a single default route on the hosts. The VRRP routing platforms share the IP address corresponding to the default route configured on the hosts. At any time, one of the VRRP routing platforms is the master (active) and the others are backups. If the master fails, one of the backup routers or switches becomes the new master router, providing a virtual default routing platform and enabling traffic on the LAN to be routed without relying on a single routing platform. Using VRRP, a backup router can take over a failed default router within a few seconds. This is done with minimum VRRP traffic and without any interaction with the hosts.

Routers or running VRRP dynamically elect master and backup routers. You can also force assignment of master and backup routers using priorities from 1 through 255, with 255 being the highest priority. In VRRP operation, the default master router sends advertisements to backup routers at regular intervals. The default interval is 1 second. If a backup router does not receive an advertisement for a set period, the backup router with the next highest priority takes over as master and begins forwarding packets.

Note: To minimize network traffic, VRRP is designed in such a way that only the router that is acting as the master sends out VRRP advertisements at any given point in time. The backup routers do not send any advertisement until and unless they take over mastership.

VRRP for IPv6 provides a much faster switchover to an alternate default router than IPv6 Neighbor Discovery (ND) procedures. Typical deployments use only one backup router.

Figure 1 illustrates a basic VRRP topology. In this example, Routers A, B, and C are running VRRP and together they make up a virtual router. The IP address of this virtual router is 10.10.0.1 (the same address as the physical interface of Router A).

Figure 1: Basic VRRP

Because the virtual router uses the IP address of the physical interface of Router A, Router A is the master VRRP router, while Routers B and C function as backup VRRP routers. Clients 1 through 3 are configured with the default gateway IP address of 10.10.0.1. As the master router, Router A forwards packets sent to its IP address. If the master virtual router fails, the router configured with the higher priority becomes the master virtual router and provides uninterrupted service for the LAN hosts. When Router A recovers, it becomes the master virtual router again.

VRRP is defined in RFC 3768, Virtual Router Redundancy Protocol (VRRP). VRRP for IPv6 is defined in Internet draft draft-ietf-vrrp-ipv6-spec-08.txt, Virtual Router Redundancy Protocol for IPv6. See also Internet draft draft-ietf-vrrp-unified-mib-06.txt, Definitions of Managed Objects for the VRRP over IPv4 and IPv6.

Note: Even though VRRP, as defined in RFC 3768, does not support authentication, the Junos OS implementation of VRRP supports authentication as defined in RFC 2338. This support is achieved through the backward compatibility options in RFC 3768.