Open Shortest Path First

Open Shortest Path First (OSPF) is classified as an Interior Gateway Protocol (IGP). This means that it distributes routing information between routers belonging to a single autonomous system (AS), a group of routers all using a common routing protocol. The OSPF protocol is based on link-state or shortest path first (SPF) technology. It has been designed expressly for the TCP/IP Internet environment, including explicit support for IP subnetting and the tagging of externally derived routing information.

OSPF performs the following tasks:

Multiple routes: Provides support for up to 16 equal-cost routes.
Authentication: Provides for the authentication of routing updates.
IP multicast: Uses IP multicast when sending or receiving the updates.
Allows network grouping: Allows sets of networks to be grouped together. Such a grouping is called an area. The topology of an area is hidden from the rest of the autonomous system. This method of hiding information enables a significant reduction in routing traffic. Also, routing within the area is determined only by the area's own topology, lending the area protection from bad routing data. An area is a generalization of an IP subnetted network.
IP subnet configuration: Enables the flexible configuration of IP subnets. Each route distributed by OSPF has a destination and mask. Two different subnets of the same IP network number may have different sizes (that is, different masks). This is commonly referred to as variable length subnetting. A packet is routed to the best (longest or most specific) match. Host routes are considered to be subnets whose masks are all ones (0xFFFFFFFF).
Authenticate OSPF protocol exchanges: Can be configured such that all OSPF protocol exchanges are authenticated. This means that only trusted routers can participate in the autonomous system's routing. A single authentication scheme is configured for each physical link. This enables some links to use authentication while others do not.

OSPF is a dynamic routing protocol. It quickly detects topological changes in the AS (such as router interface failures) and calculates new loop-free routes after a period of convergence. This period of convergence is short and involves a minimum of routing traffic as compared to the RIP protocol.

In a link-state routing protocol, each router maintains a database describing the autonomous system's topology. Each individual piece of this database is a particular router's local state (for example, the router's usable interfaces and reachable neighbors). The router distributes its local state throughout the autonomous system by flooding.

To generate routes, all routers run the exact same algorithm, in parallel. From the topological database, each router constructs a tree of shortest paths with itself as root. This shortest-path tree gives the route to each destination in the autonomous system. Externally derived routing information (for example, routes learned from the RIP protocol) appears on the tree as leaves. When multiple equal-cost routes to a destination exist, the routes (up to 16) are added to the TCP/IP stack's route table. The TCP/IP stack uses these equal-cost routes according to the multipath setting configured for the route table. Multiple equal-cost routes provides additional information about the multipath setting configured for a route table and the use of multiple equal-cost routes.

Externally derived routing data (for example, routes learned from the RIP protocol) is passed transparently throughout the autonomous system. This externally derived data is kept separate from the OSPF protocol's link state data. Each external route can also be tagged by the advertising router, but not by OMPROUTE, enabling the passing of additional information between routers on the boundaries of the autonomous system. OMPROUTE does pass tags created by others. For information on configuring OSPF, see Steps for configuring OSPF and RIP (IPv4 and IPv6).