OSPF Packet Format

All OSPF packets begin with a 24-byte header, as illustrated in Figure

The following descriptions summarize the header fields illustrated in Figure 46-2.

•Version number—Identifies the OSPF version used.

•Type—Identifies the OSPF packet type as one of the following:

–Hello—Establishes and maintains neighbor relationships.

–Database description—Describes the contents of the topological database. These messages are exchanged when an adjacency is initialized.

–Link-state request—Requests pieces of the topological database from neighbor routers. These messages are exchanged after a router discovers (by examining database-description packets) that parts of its topological database are outdated.

–Link-state update—Responds to a link-state request packet. These messages also are used for the regular dispersal of LSAs. Several LSAs can be included within a single link-state update packet.

–Link-state acknowledgment—Acknowledges link-state update packets.

•Packet length—Specifies the packet length, including the OSPF header, in bytes.

•Router ID—Identifies the source of the packet.

•Area ID—Identifies the area to which the packet belongs. All OSPF packets are associated with a single area.

•Checksum—Checks the entire packet contents for any damage suffered in transit.

•Authentication type—Contains the authentication type. All OSPF protocol exchanges are authenticated. The authentication type is configurable on per-area basis.

•Authentication—Contains authentication information.

•Data—Contains encapsulated upper-layer information.

OSPF ( Open Shortest Path First )

Open Shortest Path First (OSPF) is a routing protocol developed for Internet Protocol (IP) networks by the Interior Gateway Protocol (IGP) working group of the Internet Engineering Task Force (IETF). The working group was formed in 1988 to design an IGP based on the Shortest Path First (SPF) algorithm for use in the Internet. Similar to the Interior Gateway Routing Protocol (IGRP), OSPF was created because in the mid-1980s, the Routing Information Protocol (RIP) was increasingly incapable of serving large, heterogeneous internetworks.

OSPF is a link-state routing protocol that calls for the sending of link-state advertisements (LSAs) to all other routers within the same hierarchical area. Information on attached interfaces, metrics used, and other variables is included in OSPF LSAs. As OSPF routers accumulate link-state information, they use the SPF algorithm to calculate the shortest path to each node.

OSPF was derived from several research efforts, including Bolt, Beranek, and Newman's (BBN's) SPF algorithm developed in 1978 for the ARPANET (a landmark packet-switching network developed in the early 1970s by BBN), Dr. Radia Perlman's research on fault-tolerant broadcasting of routing information (1988), BBN's work on area routing (1986), and an early version of OSI's Intermediate System-to-Intermediate System (IS-IS) routing protocol.

OSPF has two primary characteristics.
The first is that the protocol is open, which means that its specification is in the public domain. The OSPF specification is published as Request For Comments (RFC) 1247.
The second principal characteristic is that OSPF is based on the SPF algorithm, which sometimes is referred to as the Dijkstra algorithm, named for the person credited with its creation.