2.7 One Backup Protecting Two Masters
Since we have covered 1-to-1 and 1-to-N redundancies, it may be informative to consider how to implement an N-to-1 redundancy using VRRP. To illustrate this case, we install three VRRP routersR1, R2, and R3into our local networks consisting of two segments. We would like R2 to back up both R1 and R3. To achieve this, we define two different virtual routers, V1 and V2, and designate R1 as the master of V1 and R3 as the master of V2. The VRRP router R2 assumes the role of the backup both in V1 and V3. Figure 2-8 illustrates this configuration.
FIGURE 2-8. One backup for two masters
Note that in Figure 2-8, at the failure of R1, R2 assumes the responsibility of handling IP(R1), and if during that period R3 also fails, R2 also starts handling packets forwarded toward IP(R2). It goes without saying that this arrangement may lead to unacceptable service degradation unless the network is overengineered and/or R2 is a mighty powerful box.
Also note that the two virtual routers in our illustration are associated with two LAN segments. R1 is on A and R3 is on B, whereas R2 is associated with both through its two interfaces: R2.1 and R2.2.
Another point to consider is that in this configuration the router backing up two masters is not protected. In the event of its failure, both R1 and R2 become unprotected. The very structure of N-to-1 leads to this shortcoming. On the other hand, this configuration is not uncommon and without some merits; given the cost considerations for backup lines, from an economic point of view, N-to-1 arrangements may be quite plausible.