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Layer 3—VRRP Router Redundancy

The Virtual Router Redundancy Protocol (VRRP) was designed to remove a single point of failure where hosts connected to the rest of the enterprise network or Internet through one default router. The VRRP is based on an election algorithm, where there are two routers: one master who owns both a MAC and IP address, and a backup, on one LAN or VLAN segment. The hosts all point to one IP address that points to the master router. The master and backup constantly send multicast messages to each other. Depending on the vendor-specific implementation, the backup will assume the master role if the master is no longer functioning or has lowered in priority based on some criteria. The new master also assumes the same MAC address, so that the clients do not need to update their ARP caches.

The VRRP, by itself, has left open many aspects so that switch manufacturers can implement and add features to differentiate themselves. All vendors offer a variety of features that alter the priority, which can be tied to server health checks, number of active ports, and so on. Whichever router has the highest priority becomes the master. These configurations need to be closely monitored to prevent oscillations. Often, a switch is configured to be too sensitive causing it to constantly change priority, hence, fluctuating from master to backup.

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