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

RIP-2 Version-Specific Features

RIP-2 represents a very modest change to the basic Routing Information Protocol. RIP-2 works in the same basic way as RIP-1 (part of why I was able to describe the operation of both in the same general section.) In fact, the new features introduced in RIP-2 are described as extensions to the basic protocol, conveying the fact that they are layered upon regular RIP-1 functionality. The five key RIP-2 extensions are:

• Classless Addressing Support and Subnet Mask Specification: When RIP-1 was developed, the use of subnets in IP (as described in RFC 950) had not yet been formally defined. It was still possible to use RIP-1 with subnets, through the use of a heuristic to determine if the destination is a network, subnet or host. However, there was no way to clearly specify the subnet mask for an address using RIP-1 messages.
  
  RIP-2 adds explicit support for subnets by allowing a subnet mask within the route entry for each network address. It also provides support for variable-length subnet masking (VLSM) and classless addressing (CIDR).
  
  
• Next Hop Specification: In RIP-2, each RIP entry includes a space where an explicit IP address can be entered as the next hop router for datagrams intended for the network in that entry. This feature can help improve efficiency of routing by eliminating unnecessary extra hops for datagrams sent to certain destinations.
  
  One common use of this field is when the most efficient route to a network is through a router that is not running RIP. Such a router will not exchange RIP messages and would therefore not normally be selected by RIP routers as a next hop for any network. The explicit Next Hop field allows the router to be selected as the next hop regardless of this situation.
  
  
• Authentication: RIP-1 included no authentication mechanism, which is a problem because it could potentially allow a malicious host to muck up an internetwork by sending bogus RIP messages around. RIP-2 provides a basic authentication scheme, which allows routers to ascertain the identity of a router before it will accept RIP messages from it.
  
  
• Route Tag: Each RIP-2 entry includes a Route Tag field, where additional information about a route can be stored. This information is propagated along with other data about the route as RIP entries are sent around the internet. A common use of this field is when a route is learned from a different autonomous system, to identify the autonomous system from which the route was obtained.
  
  
• Use of Multicasting: To help reduce network load, RIP-2 allows routers to be configured to use multicasts instead of broadcasts for sending out unsolicited RIP Response messages. These datagrams are sent out using the special reserved multicast address 224.0.0.9. All routers on an internetwork must obviously use multicast if this is to work properly.

As you can see, many of these extensions require more information to be included with each advertised route. This is where all that “extra space” in the message format of RIP-1 routing entries comes in handy, as we will see shortly.

RIP-2 messages are exchanged using the same basic mechanism as RIP-1 messages. Two different message types exist, RIP Request and RIP Response. They are sent using the User Datagram Protocol (UDP) using UDP reserved port number 520. The semantics for the use of this port is the same as for RIP-1. For convenience, I repeat the description here:

• RIP Request messages are sent to UDP destination port 520. They may have a source port of 520 or may use an ephemeral port number.
  
  
• RIP Response messages sent in reply to an RIP Request are sent with a source port of 520, and a destination port equal to whatever source port the RIP Request used.
  
  
• Unsolicited RIP Response messages (sent on a routine basis and not in response to a request) are sent with both the source and destination ports set to 520.

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