Routing in an IS-IS Network
When a group of end systems (hosts) and intermediate systems (routers) with the same area IDs in their NSAPs are connected together, they begin forming adjacencies using ES-IS and IS-IS.
Hosts rely on the nearest L1 router within their area to forward all traffic for them unless they are redirected. A router may use ES-IS either to tell a host to send its packets for a given destination to another L1 router, or to tell a host to send its packets directly to the receiving ES (if they are on the same physical link).
Hosts send any traffic with a destination outside the area to the nearest L2 router, which examines its database to find a path to another L2 router within that area and forwards the traffic.
L1 routers ...
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