开放式最短路径优先

本條目仍然有文字尚未被翻譯成中文，條目是根據其他語言的維基百科內容進行翻譯。 歡迎您積極參與翻譯與修訂。

开放式最短路径优先（英文Open Shortest Path First，OSPF）是一种链路状态，分层内部网关协议（IGP）的路由协议。著名的迪杰斯特算法被用来计算最短路径树。它使用“成本”作为路由度量。链路状态数据库用网络拓扑来创建，它在区域中的全部路由器上是等价的。

OSPF协议大概是在大型网络上使用最为广泛的IGP协议。节点在建立邻接，接受连接状态宣告(link-state advertisement)之前，可以通过MD5进行安全验证。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.

一个OSPF网络可以被划分成更小的网络。其中，一个特别的区域(area)被称为骨干区域(backbone area),该区域是整个OSPF网络的核心区域，并且所有其他的区域都与之连接。所有的内部路由都通过骨干区域。所有的区域都必须连接到骨干区域，如果不能建立直接连接，那么可以通过虚拟通道建立虚拟连接(virtual link)。

同一个广播域(broadcast domain)的路由器或者一个点对点(point to point)连接的两端的路由器，在发现彼此的时候，建立邻接(adjacencies)(译注：此句从英文维基翻译，但内容似有疑问，实际上，广播网路中的路由器彼此之间并不一定建立邻接，这取决于网络中指定路由器的选举结果,所有的路由器都与同一个广播网络中的指定路由器以及备份指定路由器建立邻接，而非指定路由器之间彼此仅仅纪录对方为双工通信可能而不建立邻接。)。同一广播网络或者模拟广播网络的点对点网络中的路由器会选举指定路由器(designated router, DR)和备份指定路由器(backup designated router, BDR)，DR和BDR作为网络的中心负责路由器之间的信息交换从而降低了网络中的信息流量。OSPF协议同时使用单播(unicast)和多播(multicast)来发送Hello包和连接状态更新(link state updates)，使用的多播地址为224.0.0.5和224.0.0.6。与RIP和BGP不同的是，OSPF协议不使用TCP或者UDP协议而是直接使用IP协议89。

[编辑] 区域类型

一个OSPF网络被分割成多个区域。区域将网络中的路由器在逻辑上分组并以区域为单位向网络的其余部分发送汇总路由信息。OSPF协议规定了几个"特殊"的区域类型。

[编辑] 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.

[编辑] Stub area

A stub area is an area which doesn't receive external routes. External routers 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.

[编辑] 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, but within certain limits

[编辑] 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.

[编辑] 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.

[编辑] 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 another ASs throughout its own AS.

[编辑] Internal Router

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

[编辑] 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.

[编辑] 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.
• 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.

[编辑] 分层

TCP/IP参考模型分为四层：应用层（Application Layer）、传输层（Transport Layer）、网络层（Internet Layer）、链路层（Link Layer）。

[编辑] External links

• IETF OSPF Working Group
• Zebra free router software which supports OSPF
• Quagga a fork of Zebra