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 1 (RIP-1) Message Format 1 2 3 RIPng ("RIPv6") Message Format and Features

RIP-2 Message Format

The basic message format for RIP-2 is also pretty much the same as it was for RIP-1, with the Version field of course set to 2, to clearly identify the message as being RIP-2. The real differences are in the individual RIP entries, as you can see in Table 121 and Figure 177.

Table 121: RIP Version 2 (RIP-2) 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.
Version	1	Version Number: Set to 2 for RIP version 2.
Must Be Zero	2	Reserved: Field reserved; value must be set to all zeroes.
Route Table Entries (RTEs)	20 to 500, in increments of 20

As you can see, the unused fields allow the new RIP-2 features to be implemented without changing the basic structure of the RIP entry format. This allows RIP-1 and RIP-2 messages and devices to coexist in the same network. A RIP-2 device can handle both RIP-1 and RIP-2 messages, and will look at the version number to see which version the message is. A RIP-1 device should handle both RIP-2 and RIP-1 messages the same way, simply ignoring the extra RIP-2 fields it doesn't understand.

RIP Version 1 (RIP-1) Message Format 1 2 3 RIPng ("RIPv6") Message Format and Features