Open Shortest Path First (OSPF) is a link-state, 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. The Internet protocol suite is the set of communications protocols that implement the protocol stack on which the Internet runs. ... The application layer is the seventh level of the seven-layer OSI model. ... The File Transfer Protocol (FTP) is a software standard for transferring computer files between machines with widely different operating systems. ... HTTP (for HyperText Transfer Protocol) is the primary method used to convey information on the World Wide Web. ... HTTPS is the secure version of HTTP, the communication protocol of the World Wide Web. ... The Internet Message Access Protocol (commonly known as IMAP, and previously called Interactive Mail Access Protocol) is an application layer Internet protocol used for accessing email on a remote server from a local client. ... Internet Relay Chat (IRC) is a form of instant communication over the Internet. ... The Network News Transfer Protocol or NNTP is an Internet application protocol used primarily for reading and posting Usenet articles, as well as transferring news among servers. ... Post Office Protocol version 3 (POP3) is an application layer Internet standard protocol used to retrieve email from a remote server to a local client over a TCP/IP connection. ... Session Initiation Protocol (SIP) is a protocol developed by the IETF MMUSIC Working Group and proposed standard for initiating, modifying, and terminating an interactive user session that involves multimedia elements such as video, voice, instant messaging, online games, and virtual reality. ... 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OSPF is perhaps the most widely used IGP 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 RIP, it was VLSM capable or classless from its inception. A newer version of OSPF (OSPFv3) now supports IPv6 as well. Multicast extensions to OSPF (MOSPF) have been defined, however these are not widely used. OSPF can "tag" routes, and propagate these tags along with the routes. Internal Gateway Protocol (IGP) refers to a routing protocol that is used within an autonomous system. ... This article is chiefly about the Routing Information Protocol for IPv4 and IPv6. ... Classless Inter-Domain Routing (CIDR), introduced starting in 1993, is the latest refinement to the way IP addresses are interpreted. ...

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 RIP or BGP, OSPF does not use TCP or UDP but uses IP directly, using IP protocol 89. A broadcast domain is a logical area in a computer network where any computer connected to the computer network can directly transmit to any other in the domain without having to go through a routing device. ... Point-to-Point telecommunications is most recently (2003) referenced regarding wireless data communications for Internet or Voice over IP via radio frequencies in the multi-gigahertz range. ... In computer networks, unicast is the sending of information packets to a single destination. ... Multicast is the delivery of information to a group of destinations simultaneously using the most efficient strategy to deliver the messages over each link of the network only once and only create copies when the links to the destinations split. ... In computer networking a multicast address is an identifier for a group of hosts that have joined a multicast group. ... This article is chiefly about the Routing Information Protocol for IPv4 and IPv6. ... The border gateway protocol (BGP) is one of the core routing protocols in the Internet. ...

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 intra-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 stub.. A Default route is the IP address used by a router when no other assigned route works for a given IP packets destination. ...

Not-so-stubby area

Also referred to as NSSA, a not-so-stubby area is a type of stub area that can import AS 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 (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. In the Internet, an autonomous system (AS) is a collection of IP networks and routers, under the control of one or more entities, that presents a common routing policy to the Internet. ...

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.

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, standard 54.
• 1999, December - OSPFv3, IPv6, RFC 2740.
• 2003, January - The OSPF NSSA Option updated, RFC 3101.

Internet standards are defined by the Internet Engineering Task Force (IETF). ...

External links

Information Sources

• IETF OSPF Working Group

Implementations

• Zebra router software for Unix-like systems which supports OSPF
• Quagga a fork of Zebra
• XORP a routing suite including a limited OSPF daemon