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7.1 | ![]() |
IS-IS Fundamentals | |
7.1.3 | ![]() |
ES-IS and IS-IS |
ISO has developed standards for two
types of network protocols used in routing. These two protocols are
ES-IS and IS-IS. The ES-IS discovery
protocols are used for routing between end systems and intermediate
systems. The IS-IS routing protocols are used for hierarchical routing
between intermediate systems.
ES-IS is an analogous to Address Resolution Protocol (ARP) in IP. Although not technically a routing protocol, ES-IS is included here because it is commonly used with routing protocols to provide end-to-end data movement through an internetwork. Routing between end systems and intermediate systems is sometimes referred to as Level 0 routing. With IS-IS routing, OSI distinguishes between Level 1, Level 2, and Level 3 routing to simplify router design and operation. 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. Level 3 routing is done between separate domains. Hierarchical routing simplifies backbone design, because Level 1 ISs only need to know how to get to the nearest Level 2 IS.
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