Identifying Systems and Establishing IS-IS Adjacencies
As you now know, an IS can be part of only one Level 2 area (as shown in Figure 6.2). It is the area ID and address that specifically identifies the area and system ID of each network node. Therefore, every IS in a particular area must use the same identifying area address (just as nodes in an IP subnet use the same subnetwork address). An adjacency is created between a host (end system) and a Level 1 router (IS) if they both have an identical area address. ESs create adjacencies only with ISs that have the same area addresses and with other ESs on their same subnetwork. Routing performed at Level 1 is based on SysID values (generally MAC addresses or an IP loopback address coded into the ...
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