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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  TCP/IP Routing Information Protocol (RIP, RIP-2 and RIPng)                           9  RIP Version-Specific Message Formats and Features

RIP Version-Specific Message Formats and Features 1 2 3 RIP Version 2 (RIP-2) Message Format and Features

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	Size (bytes)	Description
Command	1	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.
Version	1	Version Number: Set to 1 for RIP version 1.
Must Be Zero	2	Reserved: Field reserved; value must be set to all zeroes.
RIP Entries	20 to 500, in increments of 20

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.

RIP Version-Specific Message Formats and Features 1 2 3 RIP Version 2 (RIP-2) Message Format and Features