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Figure 8.8. The RIPv1 and RIPV2 message formats.<br />

RIPv1 and RIPv2 use <strong>the</strong> same message format. As you can see,<br />

RIPv2 uses <strong>the</strong> unused fields in <strong>the</strong> RIPv1 message <strong>to</strong> hold <strong>the</strong><br />

additional data. Up <strong>to</strong> 25 routes can be sent in a RIP message (<strong>to</strong>tal<br />

message size is 512 bytes). The message fields for Figure 8.8 are<br />

defined as follows:<br />

• Command—The command flag defines <strong>the</strong> message type:<br />

request or response.<br />

• Version—Specifies <strong>the</strong> version of RIP that created <strong>the</strong> message:<br />

version 1 or version 2.<br />

• Address Family Identifier—States which pro<strong>to</strong>col is using RIP for<br />

routing calculations. The ID for IP is 2.<br />

• Route Tag—This is a version 2-specific field. It is reserved for<br />

use in redistribution. RIPv1 does not use this field.<br />

• IP Address (<strong>the</strong> vec<strong>to</strong>r)—This can be an IP address of a host, IP<br />

classful <strong>network</strong> address, IP classful subnet address, or a<br />

default route.<br />

• Subnet Mask: RIPv2 only—The netmask of <strong>the</strong> IP address.<br />

• Next Hop: RIPv2 only—This is used as a hint; if an alternative<br />

same or lower hop gateway exists on <strong>the</strong> same subnet it will be<br />

sent along in <strong>the</strong> update in this field.<br />

• Metric—The distance (hop count) between <strong>the</strong> router that is<br />

sending <strong>the</strong> update and <strong>the</strong> destination <strong>network</strong>.

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