VRRP, like the ESRP, allows multiple switches to provide redundant routing services to users.
VRRP is used to eliminate the single point of failure associated with manually configuring a default gateway address on each host in a network. Without using VRRP, if the configured default gateway fails, you must reconfigure each host on the network to use a different router as the default gateway. VRRP provides a redundant path for the hosts. Using VRRP, if the default gateway fails, the backup router assumes forwarding responsibilities. An example VRRP topology is shown in Simple VRRP Network.
Switches A and B are both configured with the same VRRP router ID on the same VLAN, which establishes a VRRP relationship between the two routers. Because a single switch can support multiple VRRP relationships, each relationship is referred to as a VRRP router instance. Within a VRRP router instance, any VRRP router can become the master, but only one VRRP router can be master at a time. The master processes all client communications, and the other VRRP routers in the VRRP routing instance stand by, ready to take over if the master is no longer available.
Each switch in a VRRP topology has its own unique IP and MAC addresses, which are required for basic IP connectivity. For each VRRP router instance, there are shared VRRP IP and MAC addresses, which are used for network client communications. The VRRP router IP address is configured on all VRRP routers in a VRRP routing instance, and it is configured as the default gateway address on network clients. If the master VRRP router becomes unavailable, the backup VRRP router takes over using the same VRRP router IP address.
If the VRRP router IP address matches the actual VLAN IP address of the IP address owner has the highest priority value (255) and will always become the master when VRRP is enabled and operating correctly. If the switch or the VRRP process on the switch stops responding, a backup switch (Switch B in VRRP Load-sharing Configuration) takes over the master role and serves as the default gateway for network clients.
VRRP supports multiple backup routers. If the master VRRP router stops working, one of the backup routers takes over as described in ESRP Master Election.
VRRP also supports multiple VRRP router instances, which can be used to enable load sharing. The following figure shows a VRRP load-sharing configuration.
Switches A and B in the above figure are each configured with two VRRP router instances. Switch A is the IP address owner and default master for VRRP instance 1, and Switch B is the IP address owner and default master for VRRP instance 2. Half the network clients are configured to use VRRP instance 1 as the primary gateway and VRRP instance 2 as the backup gateway. The other half of the network clients are configured to use VRRP instance 2 as the primary gateway and VRRP instance 1 as the backup gateway. When both switches are operating with VRRP, each switch supports half the clients in a load-sharing topology. If either switch fails, or if VRRP is disabled on a switch, the remaining switch supports all network clients.
NoteWe recommend that you do not enable VRRP on aggregated VLANs, which are also known as super VLANs.