Overview of the ISO IS IS Intra Domain Routing Protocol
The IS IS Routing Protocol [1] has been developed in ISO to provide routing for pure OSI envi ronments. In particular, IS IS is designed to work in conjunction with ISO 8473 (The ISOConnectionless Network Layer Protocol [2] and ISO 9542 (The ISO End System to Intermedi ate System Protocol [3] This paper briefly describes the manner in which IS IS is used to sup port pure OSI environments. Enhancements for support of IP and multi protocol environmentshave been specified [4,5] but are beyond the scope of this paper.NOTE: The ISO term for a router is an "intermediate system" IS IS is so named because it is aprotocol which runs between routers. In other words, it is an "intermediate system to intermediatesystem" (or IS to IS) protocol. In this paper, the terms "router" "intermediate system" and "IS"are used interchangeably.IS IS provides an "intra domain" routing protocol. That is, it provides a standard for routingwithin a routing domain. The boundaries of routing domains are defined by network manage ment, by setting some links to be "external domain" If a link is marked as "external domain" noIS IS routing messages are sent on that link.ISO has not yet completed a standard for inter domain routing. The OSI Inter domain routingprotocol is currently at the "commitee draft" stage [6] and is expected to become a full standardbefore the end of 1992. However, the inter domain routing protocol is not yet widely available inproducts. Instead, manual configuration is used. The link is statically configured with the set ofaddress prefixes reachable via that link, and with the method by which they can be reached (suchas the DTE address to be dialed to reach that address, or the fact that the DTE address should beextracted from the IDP portion of the ISO address) OSI IS IS routing makes use of two level hierarchical routing. A routing domain is partitionedinto "areas" Level 1 routers know the topology in their area, including all routers and end sys tems in their area. However, level 1 routers do not know the identity of routers or destinationsoutside of their area. Level 1 routers forward all traffic for destinations outside of their area to alevel 2 router in their area. Similarly, level 2 routers know the level 2 topology, and know whichaddresses are reachable via each level 2 router. However, level 2 routers do not need to know thetopology within any level 1 area, except to the extent that a level 2 router may also be a level 1router within a single area. Only level 2 routers can exchange data packets or routing informationdirectly with external routers located outside of the routing domains. ISO provides for flexible, variable length addresses (known as Network Service Access PointAddresses, or NSAPs [7] which allow for multi level hierarchical address assignment. These addresses provide the flexibility needed to simultaneously solve two critical problems: (i) How to