Link State Routing Basics

• Microsoft Windows NT Server versions 4.0
• Microsoft Routing and Remote Access Service Update for Windows NT Server 4.0

SUMMARY

There are 2 types of distributed routing technologies. They are distance vector and link state. This article discusses link state routing.

MORE INFORMATION

Routers that use a link state routing protocol maintain a database of their individual autonomous system (AS)topology. An autonomous system is a group or collection of networks under common administration using the same routing protocol, and is sometimes called a routing domain. Autonomous systems are commonly divided into areas tied together by a backbone area. Each router in an autonomous system has an identical link state database (LSB). The link state database is composed of each router's local state.

Routers distribute their local state by flooding the autonomous system with link state advertisements (sometimes referred to as link state packets). Link state advertisements (LSAs) are special packets which contain information about neighbors and route cost. These LSAs can also contain routing information gathered by other routing protocols and static routes.

All routers in an autonomous system run the same routing algorithm. From the link state database, each router builds a tree of shortest paths with itself as the root. The tree contains the route to each destination in the autonomous system.

Common link state routing protocols are open shortest path first (OSPF) and intermediate system-to-intermediate system interdomain routing protocol (IS-IS).