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RIP General Operation, Messaging and Timers
(Page 1 of 3)
RIP is a protocol for exchanging
routing information, so its operation can best be described in terms
of the messages used to exchange this information, and the rules for
when messages are sent. The RIP software in each router sends messages
and takes other actions both in reaction to certain events and in response
to triggers set off by timers. Timers are also used to determine when
routing information should be discarded if not updated.
RIP Messages and Basic Message Types
Communication between RIP software
elements in routers on an internetwork is accomplished through the use
of RIP messages. These messages are sent using the User
Datagram Protocol (UDP), with reserved
UDP port number 520 for RIP-1 and RIP-2, and 521 for RIPng. Thus, even
though RIP is considered part of layer three like other routing protocols,
it behaves more like an application in terms of how it sends messages.
The exact format of the message is version-dependent, and all three
formats (for RIP, RIP-2 and RIPng) are described in detail in their
own section. RIP messages can be either
sent to a specific device, or can be sent out for multiple devices to
receive. If directed to one device they are sent unicast; otherwise,
they are either broadcast (in RIP) or multicast (RIP-2 and RIPng).
There are only two basic message
types for all three versions of RIP:
- RIP Request: A message sent by
a router to another router asking it to send back all or part of its
routing table.
- RIP Response: A message sent by
a router containing all or part of its routing table. Note that despite
the name, this message is not sent just in response to
an RIP Request message, as we will see. So it's not really that
great a name… oh well.
 Note: The original RIP also defined a few other message types: Traceon, Traceoff and a special message type reserved for use by Sun Microsystems. These are obsolete and no longer used, and were removed from the RIP-2 and RIPng standards.
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