Intermediate system to intermediate system

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ISO 10589 defines the intermediate system to intermediate system (IS-IS) protocol as an intradomain, interarea, link state routing protocol, based on Digital’s DECnet Phase V routing algorithm. IS-IS can operate over a variety of subnetworks including LANs, WANs, and point-to-point links. It was adopted by the ISO and is used to route connectionless network protocol (CLNP) traffic. Its architecture enables it to adapt easily to route the IP, which is documented as Integrated IS-IS. It was also adapted by Novell to route NetWare’s internetwork packet exchange (IPX) protocol and is documented as NetWare link services protocol (NLSP). The same person who designed it for Digital (Radia Perlman) adapted it for Novell.

As a link state routing protocol, IS-IS has the advantage of rapid convergence, but the disadvantage of increased complexity. IS-IS provides a hierarchical routing approach defining two levels of routing. Level 1 intermediate systems (IS) provide routing within the same area. An area is a logical portion of a routing domain or autonomous system (AS). Level 2 intermediate systems route information between areas within the routing domain that forms the routing backbone. Simply stated, Level 2 intermediate systems route between Level 1 areas. The backbone concept simplifies the design because a Level 1 IS need know only how to get to the nearest Level 2 IS when the destination resides in another area.

Each IS generates an update specifying the end systems (ES) and intermediate systems it is connected to, and an associated metric for reaching that destination. The update is then flooded to its neighbors. IS-IS uses a simple shortest path first (SPF) algorithm for determining the best route to a destination. In fact, IS-IS defines a four-level metric (bandwidth, delay, expense, probability). Link level packet authentication is also supported.