2.4 Multiple Backups
The examples we have introduced so far were of the 1-to-1 cardinality; even in the mutual protection case, we had a pair of 1-to-1 configurations. VRRP supports 1-to-N redundancy cardinalities where N can be greater than 1. To illustrate the basic elements of such configurations, let us consider a case in which the R1 router is backed up by two routers, R2 and R3, without any load sharing. Figure 2-6 represents such a configuration.
FIGURE 2-6. A virtual router with multiple backups
In this configuration R1 is designated as the default router for all the hosts and the master of the virtual router V1. R2, R3 are pure redundant backups ready to take over the default router role if R1 were to fail. This configuration calls our attention more vividly than the other cases we have studied to the fact that we need a mechanism for deciding which one of the routers, R2 or R3, should become the master if or when R1 fails. Actually, the criterion provided by VRRP for this switchover decision is no different than the one used for the initial decision for mastership. It is based on administratively assigned priorities ranging normally between 1 and 255. The value 0 has a special meaning to indicate that the current master is releasing its mastership responsibility. The higher the number, the higher the priority. At the initiation, the router with the highest priority becomes the master. The same applies to the failover situation in which the protocol elects the router with the highest priority as the master.
The protocol does not, however, prohibit the assignment of the same priority to different routers, and for that reason it specifies another criterion to be used as the tiebreaker in the case of two contending backups with the same priority. This second criterion is based on the specific IP address of a router. Each VRRP router is associated with a set of IP addresses identifying its multiple (real) interfaces. Using an algorithm (the smallest one, the first one in the list, etc.) a router selects one of its IP addresses as its primary IP address. VRRP uses this primary IP address as the tiebreaker in deciding between two VRRP routers with the same priority. In such a decision, the router with the greater primary IP address wins.