OSPF-2 Protocol Overview

Open Shortest Path First (OSPF) is the recent entry into the Internet interior routing scene. OSPF Version 2 is documented in RFC 1583. Sanctioned by the IETF, it is intended to become Internet's preferred interior routing protocol. OSPF is a link-state protocol with a complex set of options and features. Not all of these features are available on all implementations, but some of its advantages are:

• Scalability. OSPF is specifically designed to operate with larger networks. It does not impose a hop-count restriction and permits its domain to be subdivided for easier management.

• Full subnetting support. OSPF can fully support subnetting, including VLSM and non-contiguous subnets.

• Hello packets. OSPF uses small "hello" packets to verify link operation without transferring large tables. In stable networks, large updates occur only once every 30 minutes.

• TOS routing. OSPF can route packets by different criterion based on their Type Of Service (TOS) field. For example, file transfers could be routed over a satellite link while terminal I/O could avoid such high delays. This requires cooperative applications on the end systems.

• Tagged routes. Routes can be tagged with arbitrary values, easing interoperation with EGPs, which can tag OSPF routes with AS numbers.

OSPF has some disadvantages as well. Chief among them are its complexity and its demands on memory and computation. Although link-state protocols are not difficult to understand, OSPF comes to the rescue with plenty of options and features.

OSPF divides its routing domain into areas. Area 0, the backbone, is required. This divides interior routing into two levels. If traffic must travel between two areas, the packets are first routed to the backbone. This may cause non-optimal routes, since interarea routing is not done until the packet reachs the backbone. Once there, it is routed to the destination area, which is then responsible for final delivery. This layering permits addresses to be consolidated by area, reducing the size of databases. Small networks can operate with a single OSPF area.