Open Shortest Path First

An OSPF network can be broken up into smaller networks where a backbone always known as area zero can have any other arbitrarily numbered areas connecting directly to it. All areas must connect via area zero. If areas haven't direct physical connection to the backbone area, Virtual links must be used to connect them. It was often a general rule not to have more than fifty routers in one area. However, improvements in processor power can now allow over 400 sufficiently powerful routers to participate with no stability issues. Routers at the edge of two areas are known as 'Area Border Routers' or ABRs. Routes from other autonomous systemss are inserted into OSPF via Autonomous System Boundary Routers (ASBRs).

Routers in the same broadcast domain or at each end of a point to point link form adjacencies when they have discovered each other. The routers elect a designated router (DR) and backup designated router (BDR) which act as hub to reduce traffic between routers. OSPF uses both unicast and multicast to send 'hello packets' and link state updates. Multicast addresses 224.0.0.5 and 224.0.0.6 are used. In contrast to RIP using UDP on port 520 or BGP using TCP on port 179, OSPF does not use a Layer 4 protocol but uses IP directly, using IP protocol 89.

OSPF can utilise authentication for forming adjacencies and exchanging routing information. OSPF was VLSM capable or classless from its inception. OSPF is capable of tagging external routes.

OSPF is the suggested successor to RIP in the Internet environment.

A new version of OSPF to support IP version 6 has been proposed, commonly called OSPFv3.

Multicast extensions to OSPF (MOSPF) was a largely unsuccessful (in terms of deployment success, not in technical terms) attempt to adapt OSPF for use as multicast routing protocol.

RFC history