Best Practice: Zone Stability in Second-Tier Networks

In some topologies, a communication link failure can remove an entire zone from a second-tier network. For example, in the straightforward topology (left in Border Router: Zone Stability), a WAN failure disconnects the entire Remote Zone from border router BR. The border router automatically assesses the situation and rebuilds its zone map, however, this process can disrupt message flow for several minutes. A similar disruption can occur when the WAN resumes normal operation.

Contrast the better topology (right in Border Router: Zone Stability). BR and the first-tier router R1 both reside on the same side of the WAN link (possibly even on the same host computer). In this topology, WAN failure does not disconnect the entire Remote Zone, since BR remains in contact with R1. BR need not reconfigure its zone map, so it avoids associated delays.

Figure 82: Border Router: Zone Stability