Routing packets through an internetwork is a complex task. When a network is small and contains only a few paths to a destination, configuring static routes is usually the best solution.

However, in a multi-protocol enterprise network with hundreds of possible routes, static routes will not suffice. In order to keep an accurate, up-to-date routing table in an enterprise network, you need a robust, efficient, scalable link state routing protocol. Open Shortest Path First protocol (OSPF) meets all of these requirements, and then some.

The Internet Engineering Task Force (IETF) designed OSPF to be a scalable, efficient, and quickly converging routing protocol. Because the IETF designed OSPF, it is an open standard routing protocol and can run on routers from a variety of vendors, including Cisco, Lucent, and Nortel. As a result, if you oversee a large enterprise network supported by products from multiple vendors, you can use a single routing protocol. This approach will help ease the pains of network administration and troubleshooting.

Improved efficiency
Another potential benefit of OSPF is maximized use of bandwidth.

Convergence is the amount of time required for all the routers within an internetwork to exchange routing information. Unlike distance vector routing protocols that send their entire routing table to every router on the network every 30 seconds, OSPF sends routing updates only as changes occur in the network topology and only to routers that are directly attached. Using this process to maintain routing tables is more accurate and dramatically decreases the amount of bandwidth used for sharing route information.

OSPF can maintain thousands of routes in a routing table. Route selection is prioritized by cost. Packets are routed through an internetwork using the route with the lowest cost. To calculate the cost of a route, OSPF uses a bandwidth-based formula.

This formula is 10 to the 8th, divided by bandwidth. The bandwidth on an individual interface is usually configurable by the administrator, and bandwidth settings enable OSPF to choose the fastest route.

Further, OSPF can balance loads between multiple links. This means that when two links of equal bandwidth exist, OSPF can monitor these links and route traffic over both while monitoring link speed and saturation levels.

Specifics on how to configure OSPF vary by vendor and equipment type. However, for information on configuring OSPF on Cisco routers, check out CCIE Professional Development: Routing TCP/IP, Volume I .

Warren Heaton CCDA, CCNA, MCSE+I is the Cisco Program Manager for A Technological Advantage in Louisville, KY.

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