OSPF and IS-IS: Choosing an IGP for Large-Scale Networks

7.2. Area Reliability

A reliable area is one that can sustain the failure of one or more of its components without isolating a large segment of the network. The more failures the area can tolerate, the more robust the area.

Large network segments can be isolated in one of two ways:

• The failure of a router connecting two areas isolates an area from the rest of the network (Figure 7.2).

  Figure 7.2. A failure of one of the ABRs will isolate its attached nonbackbone area from the rest of the network.

  

• The failure of a router or link partitions an area (Figure 7.3).

  Figure 7.3. A failure of any one of the three links connecting the ABRs across area ...