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OSPF General Operation and Message Types
(Page 1 of 2)
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
OSPF Message Types
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:
- 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.
- 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.
- 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.
- 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.
- Link State Acknowledgment:
These messages provide reliability to the link-state exchange process,
by explicitly acknowledging receipt of a Link State Update message.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
© Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
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