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| 4.1 |  | RIP v2 Overview |
| 4.1.3 |  | RIP v2 message format |

RIP v2 uses a special message format, or packet, to collect and share information about distances to known internetworked destinations. The RIP v2 message format is shown in Figure . The basic structure is the same as for RIP v1. All the extensions to the original protocol are carried in the unused fields. RIP v2 updates can contain entries for up to 25 routes as in RIP v1. Like RIP v1, RIP v2 operates from UDP port 520, has an 8-byte header, and a maximum datagram size of 512 bytes.

The Command field remains unchanged from RIP v1. It indicates whether the RIP v2 route was generated as a request, or a response to a request.

The Version field will be set to two for RIP v2. If it is set to zero or one and the message is not a valid RIP v1 format, the message will be discarded. RIP v2 will process valid RIP v1 messages.

The Address Family Identifier (AFI) field is set to two for IP. The only exception is a request for a full routing table of a router or host, in which case it will be set to zero.

The Route Tag field provides a way to differentiate between internal and external routes. An internal route is one that was learned by the RIP v2 protocols within the network or autonomous system. External routes are those that were learned from other routing protocols that have been redistributed into the RIP v2 process. One suggested use of this 16-bit field is to carry the autonomous system number of routes that have been imported from an external routing protocol.

The IP Address field contains the destination address. It may be a major network address, a subnet, or a host route.

The Subnet Mask field contains a 32-bit mask that identifies the network and subnet portion of the IP address. The addition of this field is the single most important change made to the RIP v2 message structure.

The Next Hop field contains the IP address of the next hop listed in the IP Address field.

Metric indicates how many internetwork hops or routers have been traversed in the trip to the destination. This value is between 1 and 15 for a valid route, or 16 for an unreachable route.

| Interactive Media Activity Drag and Drop: RIP v2 Packet Upon completion of this activity, the student will be able to identify the different fields in a RIP v2 packet.