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Open Shortest Path First (OSPF) is a routing protocol for Internet Protocol (IP) networks. It uses a link state routing algorithm and falls into the group of interior routing protocols, operating within a single autonomous system (AS). It is defined as OSPF Version 2 in RFC 2328 (1998) for IPv4. The updates for IPv6 are specified as OSPF Version 3 in RFC 5340 (2008).〔 〕 OSPF is perhaps the most widely used interior gateway protocol (IGP) in large enterprise networks. Intermediate System to Intermediate System (IS-IS), another link-state dynamic routing protocol, is more common in large service provider networks. The most widely used exterior gateway protocol is the Border Gateway Protocol (BGP), the principal routing protocol between autonomous systems on the Internet. ==Overview== OSPF is an interior gateway protocol (IGP) for routing Internet Protocol (IP) packets solely within a single routing domain, such as an autonomous system. It gathers link state information from available routers and constructs a topology map of the network. The topology is presented as a routing table to the Internet Layer which routes datagrams based solely on the destination IP address found in IP packets. OSPF supports Internet Protocol Version 4 (IPv4) and Internet Protocol Version 6 (IPv6) networks and features variable-length subnet masking (VLSM) and Classless Inter-Domain Routing (CIDR) addressing models. OSPF detects changes in the topology, such as link failures, and converges on a new loop-free routing structure within seconds. It computes the shortest path tree for each route using a method based on Dijkstra's algorithm, a shortest path first algorithm. The OSPF routing policies for constructing a route table are governed by link cost factors (''external metrics'') associated with each routing interface. Cost factors may be the distance of a router (round-trip time), data throughput of a link, or link availability and reliability, expressed as simple unitless numbers. This provides a dynamic process of traffic load balancing between routes of equal cost. An OSPF network may be structured, or subdivided, into routing ''areas'' to simplify administration and optimize traffic and resource utilization. Areas are identified by 32-bit numbers, expressed either simply in decimal, or often in octet-based dot-decimal notation, familiar from IPv4 address notation. By convention, area 0 (zero), or 0.0.0.0, represents the core or ''backbone'' area of an OSPF network. The identifications of other areas may be chosen at will; often, administrators select the IP address of a main router in an area as area identification. Each additional area must have a direct or virtual connection to the OSPF backbone area. Such connections are maintained by an interconnecting router, known as ''area border router'' (ABR). An ABR maintains separate link state databases for each area it serves and maintains summarized routes for all areas in the network. OSPF does not use a TCP/IP transport protocol, such as UDP or TCP, but encapsulates its data in IP datagrams with protocol number 89. This is in contrast to other routing protocols, such as the Routing Information Protocol (RIP) and the Border Gateway Protocol (BGP). OSPF implements its own error detection and correction functions. OSPF uses multicast addressing for route flooding on a broadcast domain. For non-broadcast networks, special provisions for configuration facilitate neighbor discovery.〔 OSPF multicast IP packets never traverse IP routers (never traverse Broadcast Domains), they never travel more than one hop. OSPF is therefore a Link Layer protocol in the Internet Protocol Suite. OSPF reserves the multicast addresses 224.0.0.5 (IPv4) and FF02::5 (IPv6) for all SPF/link state routers (AllSPFRouters) and 224.0.0.6 (IPv4) and FF02::6 (IPv6) for all Designated Routers (AllDRouters), as specified in RFC 2328 and RFC 5340.〔(【引用サイトリンク】title=RFC 5340 - OSPF for IPv6 )〕 For routing multicast IP traffic, OSPF supports the Multicast Open Shortest Path First protocol (MOSPF) as defined in RFC 1584.〔RFC 1584, ''Multicast Extensions to OSPF'', J. Moy, The Internet Society (March 1994)〕 Cisco does not include MOSPF in their OSPF implementations. PIM (Protocol Independent Multicast) in conjunction with OSPF or other IGPs, is widely deployed. The OSPF protocol, when running on IPv4, can operate securely between routers, optionally using a variety of authentication methods to allow only trusted routers to participate in routing. OSPFv3, running on IPv6, no longer supports protocol-internal authentication. Instead, it relies on IPv6 protocol security (IPsec). OSPF version 3 introduces modifications to the IPv4 implementation of the protocol.〔 Except for virtual links, all neighbor exchanges use IPv6 link-local addressing exclusively. The IPv6 protocol runs per link, rather than based on the subnet. All IP prefix information has been removed from the link-state advertisements and from the ''Hello'' discovery packet making OSPFv3 essentially protocol-independent. Despite the expanded IP addressing to 128-bits in IPv6, area and router Identifications are still based on 32-bit values. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Open Shortest Path First」の詳細全文を読む スポンサード リンク
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