Overview
Typically, you can configure a default gateway for every host on a LAN. All packets destined for other networks are sent through the default gateway. As shown in Figure 29, when the default gateway fails, no hosts can communicate with external networks.
Figure 29: LAN networking
Using a default gateway facilitates your configuration but requires high availability. Using more egress gateways improves link availability but introduces the problem of routing among the egresses.
Virtual Router Redundancy Protocol (VRRP) is designed to address this issue. VRRP adds a group of network gateways to a VRRP group called a virtual router. A VRRP group comprises one master and multiple backups, but has only one virtual IP address. The hosts on the subnet only need to configure this virtual IP address as their default network gateway for communicating with external networks.
The virtual IP address of the virtual router can be either an unused IP address on the subnet where the VRRP group resides or the IP address of an interface on a router in the VRRP group. In the latter case, the router is called the IP address owner. A VRRP group can have only one IP address owner.
VRRP avoids single points of failure and simplifies the configuration on hosts. When the master in the VRRP group on a multicast or broadcast LAN (for example, an Ethernet network) fails, another router in the VRRP group takes over. The switchover is complete without causing dynamic route recalculation, route re-discovery, gateway reconfiguration on the hosts, or traffic interruption.
VRRP operates in either of the following modes:
Standard mode—Implemented based on RFCs. For more information, see "VRRP standard mode."
Load balancing mode—Extends the VRRP standard mode to distribute load across VRRP group members. For more information, see "VRRP load balancing mode."
VRRP has two versions: VRRPv2 and VRRPv3. VRRPv2 supports IPv4 VRRP. VRRPv3 supports IPv4 VRRP and IPv6 VRRP.