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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  TCP/IP Routing Protocols (Gateway Protocols)                 9  TCP/IP Interior Routing Protocols (RIP, OSPF, GGP, HELLO, IGRP, EIGRP)                      9  Open Shortest Path First (OSPF)

OSPF Route Determination Using SPF Trees 1 2 OSPF Message Formats

As a routing protocol, the main job of OSPF is to facilitate the exchange of routing information between routers. Each router in an OSPF autonomous system that runs OSPF software that is responsible for various tasks, such as setting timers to control certain activities that must occur on a regular basis, and the maintenance of important data structures, such as the link-state database (LSDB). Most importantly, each OSPF router must both generate and respond to OSPF messages. It is this messaging system that allows important routing information to be shared within an AS or area, which makes it crucial to understanding how OSPF works. So, it's worth taking a look at the message types and how they are used.

Unlike RIP, OSPF does not send its information using the User Datagram Protocol (UDP). Instead, OSPF forms IP datagrams directly, packaging them using protocol number 89 for the IP Protocol field. OSPF defines five different message types, for various types of communication:

1. Hello: As the name suggests, these messages are used as a form of greeting, to allow a router to discover other adjacent routers on its local links and networks. The messages establish relationships between neighboring devices (called adjacencies) and communicate key parameters about how OSPF is to be used in the autonomous system or area.
   
   
2. Database Description: These messages contain descriptions of the topology of the AS or area. That is, they convey the contents of the link-state database for the autonomous system or area from one router to another. Communicating a large LSDB may require several messages to be sent; this is done by having the sending device designated as a master device and sending messages in sequence, with the slave (recipient of the LSDB information) responding with acknowledgements.
   
   
3. Link State Request: These messages are used by one router to request updated information about a portion of the LSDB from another router. The message specifies exactly which link(s) about which the requesting device wants more current information.
   
   
4. Link State Update: These messages contain updated information about the state of certain links on the LSDB. They are sent in response to a Link State Request message, and also broadcast or multicast by routers on a regular basis. Their contents are used to update the information in the LSDBs of routers that receive them.
   
   
5. Link State Acknowledgment: These messages provide reliability to the link-state exchange process, by explicitly acknowledging receipt of a Link State Update message.

OSPF Route Determination Using SPF Trees 1 2 OSPF Message Formats