OSPF is a link state protocol that distributes routing information between routers belonging to a single IP domain; the IP domain is also known as an autonomous system (AS).
In a link-state routing protocol, each router maintains a database describing the topology of the AS. Each participating router has an identical database maintained from the perspective of that router.
From the LSDB (link state database), each router constructs a tree of shortest paths, using itself as the root. The shortest path tree provides the route to each destination in the AS. When several equal-cost routes to a destination exist, traffic can be distributed among them. The cost of a route is described by a single metric.
OSPF is an IGP (interior gateway protocol), as is RIP, the other common IGP. OSPF and RIP are compared in RIP.
NoteTwo types of OSPF functionality are available and each has a different licensing requirement. One is the complete OSPF functionality and the other is OSPF Edge Mode, a subset of OSPF that is described below. For specific information regarding OSPF licensing, see the Switch Engine 32.3 Feature License Requirements document.