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 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 for an area maintained from the perspective of that router.
From the LSDB, 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.
OSPFv3 supports IPv6, and uses commands only slightly modified from that used to support IPv4. OSPFv3 has retained the use of the 4-byte, dotted decimal numbers for router IDs, LSA IDs, and area IDs.
OSPFv3 is an IGP, as is the other common IGP for IPv6, RIPng. OSPFv3 and RIPng are compared in RIPng.
Note
Two types of OSPFv3 functionality are available and each has a different licensing requirement. One is the complete OSPFv3 functionality and the other is OSPFv3 Edge Mode, a subset of OSPFv3 that is described below. For specific information regarding OSPFv3 licensing, see the Switch Engine 32.7.1 Feature License Requirements document.