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OSPF (Open Shortest Path First) Routing

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Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

OSPF is a link-state routing protocol. Routers use it to collect information about how they are linked to other routers on an internetwork and to build a map of the network topology. Link-state routing is an alternative to distance-vector routing, an earlier routing protocol that is not as robust when used on large internetworks. While link-state routing requires more processing, routers can use it to gather more information about the entire internetwork, which provides more accurate routing and faster response to changes than is possible with distance-vector routing. See "Link-State Routing."

OSPF is an interior routing protocol designed for use within autonomous systems (ASs). An AS is a collection of networks, usually managed by a single authority and using a common routing strategy. An AS may be a private network or an Internet service provider network on the Internet. BGP (Border Gateway Protocol) is an exterior protocol that is used between autonomous systems, primarily on the Internet. Here are some important OSPF concepts:

This topic continues in "The Encyclopedia of Networking and Telecommunications."

OSPF Documents

Historically, one of the first mentions of a link-state algorithm on the Internet is RFC 1074 (The NSFNET Backbone SPF Based Interior Gateway Protocol, October 1988), which describes an adapted version of the IS-IS routing protocol. Link-state routing is further elaborated in RFC 1093 (The NSFNET Routing Architecture, February 1989). Other RFCs that reference IS-IS include RFC 1142 (OSI IS-IS Intra-domain Routing Protocol, February 1990) and RFC 1195 (Use of OSI IS-IS for Routing in TCP/IP and Dual Environments, December 1990). Note the OSI protocols were once considered as a replacement for the Internet Protocol suite on the Internet. IS-IS is still used by some service providers. See "OSI (Open Systems Interconnection) Model."

The current OSPF protocol specifications are outlined in RFC 2328 (OSPF Version 2, April 1998). This document makes obsolete a number of previous RFCs. It also provides a brief history of the development of link-state routing technology. Some other important OSPF-related RFCs are listed next. Refer to the IETF OSPF Working Group (Web site listed on the related entries page) for a larger list of RFCs and drafts.

  • RFC 1246 (Experience with the OSPF Protocol, August 1991)

  • RFC 1370 (Applicability Statement for OSPF, October 1992)

  • RFC 1371 (Choosing a Common IGP for the IP Internet, October 1992)

  • RFC 1403 (BGP OSPF Interaction, January 1993)

  • RFC 1745 (BGP4/IDRP for IP-OSPF Interaction, December 1994)

  • RFC 1793 (Extending OSPF to Support Demand Circuits, April 1995)

  • RFC 2329 (OSPF Standardization Report, April 1998)

  • RFC 2370 (The OSPF Opaque LSA Option, July 1998)

  • RFC 2676 (QoS Routing Mechanisms and OSPF Extensions, August 1999)

  • RFC 2740 (OSPF for IPv6, December 1999)

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