VRRP Router Priority and Preemption
VRRP router priority determines the role each VRRP router plays (that is, primary or backup) and the order of ascendancy for backup routers to become a primary if the primary fails. The priority of the primary router is 255, and the priorities of the backups are lower.
In Figure 3-8, if Router A (the primary in a LAN topology) fails, VRRP must determine if one of the backups (B or C) should take over. If you configure Router B with priority 101 and Router C with the default priority of 100, VRRP selects Router B to become the primary because it has the higher priority. If you configure both Routers B and C with the default priority of 100, VRRP selects the backup with the higher IP address to become the primary. In this case, Router C will become the primary, as it has a higher IP address.
Figure 3-8 VRRP Router Priority
VRRP uses preemption to determine what happens after a VRRP backup router becomes the primary. With preemption enabled by default, VRRP switches to a backup if that backup comes online with a priority higher than the new primary. In the previous example, if Router A is the primary and fails, VRRP selects Router B (next in order of priority). If Router C comes online with a higher priority than Router B, VRRP selects Router C as the new primary, even though Router B has not failed. If you disable preemption, VRRP switches only if the original primary recovers (that is, Router A) or the new primary fails.
VRRP Load Balancing
You can configure multiple VRRP groups on a physical interface. The number of VRRP groups that a router interface can support depends on the router processing capability and memory capabilities. A single physical interface can support up to 255 VRRP groups. In a topology where multiple VRRP groups are configured on a router interface, the interface can act as a primary for one VRRP group and as a backup for one or more other VRRP groups.
Figure 3-9 shows a LAN topology in which VRRP is configured so that Routers A and B share the traffic to and from Clients 1 through 4. Routers A and B act as backups to each other if either router fails. The topology contains two virtual IP addresses for two VRRP groups that overlap. For VRRP group 1, Router A is the owner of IP address 10.0.0.1 and is the primary. Router B is the backup to Router A. Clients 1 and 2 are configured with the default gateway IP address of 10.0.0.1. For VRRP group 2, Router B is the owner of IP address 10.0.0.2 and is the primary. Router A is the backup to Router B. Clients 3 and 4 are configured with the default gateway IP address of 10.0.0.2. If Router A fails, Router B takes the role of the primary router and forwards the traffic from all the clients until Router A becomes available again.
Figure 3-9 Load Balancing and Redundancy in VRRP Topology
Table 3-3 lists various differences and similarities between VRRP and HSRP.
Table 3-3 VRRP vs. HSRP
| VRRP | HSRP |
| Open standard. | Cisco proprietary. |
| RFC 3768. | RFC 2281. |
| IP encapsulation: IP Protocol number 112. | UDP encapsulation: Port number 1985. |
| One primary router and multiple backup routers. | One active router, one standby router, and other listening routers. |
| Uses the router interface IP address or virtual IP address. | Uses a virtual IP address. |
| Primary router election is based on the highest priority or highest IP address. | Active router election is based on the highest priority or highest IP address. |
| Default priority is 100. | Default priority is 100. |
| Supports tracking, preemption, and timers. | Supports tracking, preemption, and timers. |
| Advertisements at 1-second intervals. | Hello at 3-second intervals. |
| Support for load balancing. | Support for load balancing. |
| Advertisement propagation using multicast IP address 224.0.0.18. | Hello propagation using multicast IP address 224.0.0.2 (version 1) and 224.0.0.102 (version 2). |
| Virtual MAC address is 0000.5E00.01 XX , where XX is the Virtual Router Identifier (255 total). | Virtual MAC address is 0000.0C07.AC XX , where the XX is the HSRP group number. |
| Preemption enabled by default. | Preemption disabled by default. |