Open Shortest Path First (OSPF) is a Link-state routing protocol, hierarchical Interior Gateway Protocol (IGP) Routing protocol. The well-known Dijkstra s algorithm is used to calculate the Shortest path tree. It uses cost as its routing metric. A link state database is constructed of the network topology which is identical on all Routers in the area.

OSPF is perhaps the most widely used Interior Gateway Protocol in large networks. It can operate securely, using MD5 to authenticate peers before forming adjacencies, and before accepting link-state advertisements. A natural successor to Routing Information Protocol, it was Classless Inter-Domain Routing capable or classless from its inception. A newer version of OSPF (OSPFv3) now supports IPv6 as well. Multicast extensions to OSPF (Multicast extensions to OSPF) have been defined, however these are not widely used. OSPF can tag routes, and propagate these tags along with the routes.

An OSPF network can be broken up into smaller networks. A special area called the backbone area forms the core of the network, and other areas are connected to it. Inter-area routing goes via the backbone. All areas must connect to the backbone; if no direct connection is possible, a virtual link may be established.

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 Routing Information Protocol or BGP, OSPF does not use TCP or UDP but uses IP directly, using IP protocol 89.

= Area types =

An OSPF network is divided into areas . These are logical groupings of routers whose information may be summarized towards the rest of the network. Several special area types are defined:

== Backbone area ==

The backbone area (also known as area zero ) forms the core of an OSPF network. All other areas are connected to it, and inter-area routing happens via a router connected to the backbone area.

== Stub area ==

A stub area is an area which doesn t receive external routes. External routes are defined as routes which were distributed in OSPF via another routing protocol. Therefore, stub areas typically need to rely on a default route to send traffic to routes outside the domain.

== Totally stubby area ==

This type of area is proprietary to Cisco Systems. A totally stubby area is similar to a stub area, however this area does not allow summary routes in addition to the external routes. The only way for traffic to get routed outside of the area is a default route. Because there is only one route out, system resources can be saved because there are fewer decisions being made. Usually this area is only used in a totally Cisco shop (when only Cisco Systems equipment is used in the network).

== Not-so-stubby area ==

Also referred to as NSSA, a not-so-stubby area is a type of stub area that can import autonomous system (Internet) external routes and send them to the backbone, but cannot receive AS external routes from the backbone or other areas.

= OSPF router types =

OSPF defines various router types. These are logical definitions, and a router that uses OSPF may be classified as more than one of the following types. For example, a router that is connected to more than one area, and which receives routes from a BGP process connected to another AS, is both an ABR and an ASBR.

== Area Border Router ==

An Area Border Router (ABR) is a router that connects one or more OSPF areas to the main backbone network. It is considered a member of all areas it is connected to. An ABR keeps multiple copies of the link-state database in memory, one for each area.

== Autonomous System Border Router ==

An ASBR is a router connected to more than one Autonomous system (Internet) (AS), and which exchanges routing information with routers in other ASs. ASBRs typically also run a non-IGP routing protocol, such as BGP. An ASBR is used to distribute routes received from other ASs throughout its own AS.

== Internal router ==

A router is called an internal router (IR) if it only has OSPF adjacencies with routers in the same area.

== Backbone router ==

A backbone router (BR) is a router with an interface in to the backbone area. An ABR would be a BR, although the reverse need not be true.

== Designated Router ==

A designated router (DR) is the router elected by the network by elections. This is calculated by the router s priority number and the loopback address.

= RFC history =

*1989, October - First put forward as a proposed standard as RFC 1131. *1994, The OSPF NSSA Option, RFC 1587. *1994, March - Multicast extensions to OSPF proposed as RFC 1584. *1997, July - OSPF version 2, as proposed in RFC 2178 *1998, April - OSPF version 2, updated in RFC 2328, internet standard 54. *1999, December - OSPFv3, IPv6, RFC 2740. *2003, January - The OSPF NSSA Option updated, RFC 3101.

= External links =

== Information Sources ==

== Implementations ==