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RIP Version 1 (RIP-1) Message Format
(Page 2 of 3)
RIP-1 Message Format
The basic message format for RIP-1
is described in Table 120
and illustrated in Figure 176.
Table 120: RIP Version 1 (RIP-1) Message Format
Field
Name
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Size (bytes)
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Description
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Command
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1
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Command Type: Identifies
the type of RIP message being sent. A value of 1 indicates an RIP
Request, while 2 means an RIP Response. Originally, three
other values and commands were also defined: 3 and 4 for the Traceon
and Traceoff commands, and 5 reserved for use by Sun Microsystems.
These are obsolete and no longer used.
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Version
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1
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Version
Number: Set to 1 for RIP version 1.
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Must Be
Zero
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2
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Reserved: Field
reserved; value must be set to all zeroes.
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RIP
Entries
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20
to 500, in increments of 20
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Figure 176: RIP Version 1 (RIP-1) Message Format The RIP-1 message format can contain up to 25 RIP Entries. Here, RIP Entry #1 is shown here with each of its constituent subfields.
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If you're like me, the
first thing that comes to mind looking at this message format is this:
what's with all the extra space? I mean, we have four different fields
that are reserved (must be zero), and even most of the other fields
are larger than they need to be (a metric of 1 to 16 needs only 4 bits,
not 32.) The command type and version number could easily have been
made only 4 bits each as well (if not less.) And why bother having a
two-byte field to identify the address type when we are only going to
deal with IP addresses anyway?
This seeming wastefulness is actually
an artifact of the generality of the original RIP design. The protocol
was intended to be able to support routing for a variety of different
internetworking protocols, not just IP. (Remember that it wasn't even
originally developed with IP in mind.) So, the Address Family Identifier
was included to specify address type, and RIP entries were made large
enough to handle large addresses. IP only requires 4 bytes per address
so some of the space is not used.
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Version 3.0 - Version Date: September 20, 2005
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