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Table Of Contents  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)

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OSPF Route Determination Using SPF Trees
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OSPF Message Formats
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OSPF General Operation and Message Types
(Page 2 of 2)

OSPF Messaging and General Operation

The use of these messages is approximately as follows. When a router first starts up it will send a Hello message out to see if any neighboring routers are around running OSPF, and it will also send them out periodically to discover any new neighbors that may show up. When an adjacency is set up with a new router, Database Description messages will then be sent to initialize the router's LSDB.

Routers that have been initialized enter a steady state mode. They will each routinely “flood” their local networks with Link State Update messages, advertising the state of their links. They will also send out updates when they detect a change in topology that needs to be communicated. They will of course receive Link State Update messages sent by other devices, and respond with Link State Acknowledgments accordingly. Routers may also request updates using Link State Request messages.

Key Concept: The operation of OSPF involves five message types. Hello messages are used to establish contact between routers, and Database Description messages to initialize a router’s link-state database. Routine LSDB updates are sent using Link State Update messages, which are acknowledged using Link State Acknowledgments. A device may also request a specific update using a Link State Request.

When hierarchical topology is used, internal routers maintain a single LSDB and perform messaging only within an area. Area border routers have multiple LSDBs and perform messaging in more than one area. They, along with any other OSPF backbone routers, also exchange messaging information on the backbone, including summarized link-state information for the areas they border.

Again, all of this is highly simplified; the OSPF standard contains pages and pages of detailed rules and procedures governing the exact timing for sending and receiving messages.

OSPF Message Authentication

The OSPF standard specifies that all OSPF messages are authenticated for security. This is a bit misleading, however, since one of the authentication “methods” supported is “null authentication”—meaning no authentication is used. More security is provided by using the optional simple password authentication method, and the most security through the use of cryptographic authentication. These methods are described in Appendix D of RFC 2328.

Note: The Hello messages used in OSPF are also sometimes called the Hello Protocol. This is especially poor terminology because there is an actual routing protocol, described in the next section, called the HELLO Protocol. The two protocols are not related. However, I suspect that the OSPF Hello messages may have been so named because they serve a similar purpose to the messages used in the independent HELLO protocol.

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OSPF Route Determination Using SPF Trees
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OSPF Message Formats
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Version 3.0 - Version Date: September 20, 2005

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