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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
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 routers 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 authenticationmeaning 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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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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