IS-IS is an OSI link-state hierarchical routing protocol. It floods the network with link-state information to build a complete, consistent picture of network topology.

To simplify router design and operation, IS-IS distinguishes between Level 1 and Level 2 Information Services (ISs):

• Level 1 ISs communicate with other Level 1 ISs in the same area.

• Level 2 ISs route between Level 1 areas and form an intradomain routing backbone.

Hierarchical routing simplifies backbone design because Level 1 ISs only need to know how to get to the nearest Level 2 IS. The backbone routing protocol also can change without impacting the intra-area routing protocol.

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