D-Link DGS-3308FG Product Manual - Page 62

8-port Gigabit Ethernet Switch User's Guide RIP2 Message Format The message format used with RIP2 is an extension of the RIP1 format, with additional information occupying unused octets of the address field. In particular, each address includes an explicit next hop as well as an explicit subnet mask. Figure 5-18. Rip Message Format RIP 2 also attaches a 16-bit Route Tag to each entry. A router must send the same tag it receives when it transmits the route. Thus, the tag provides a way to propagate additional information such as the origin of the route. In particular, if RIP2 learns a route from another autonomous system, it can use the Route Tag to propagate the autonomous system's number. Because the version number in RIP2 occupies the same octet as in RIP1, both versions of the protocols can be used on a given router simultaneously without interference. Before processing an incoming message, RIP software examines the version number. Transmitting RIP Messages RIP messages do not contain an explicit length field or an explicit count of entries. Instead, RIP assumes that the underlying delivery mechanism will tell the receiver the length of an incoming message. In particular, when used with TCP/IP, RIP messages rely on UDP to tell the receiver the message length. RIP operates on UDP port 520. Although a RIP request can originate at other UDP ports, the destination UDP port for requests is always 520, as is the source port from which RIP broadcast messages originate. The Disadvantage of RIP Hop Counts Using RIP as an interior router protocol limits routing in two ways. First, RIP restricts routing to a hop-count metric. Second, because it uses a small value of hop count for infinity, RIP restricts the size of any network using it. In particular, RIP restricts the span of a network to 16 hops (or 15 routers, because 16 represents an unreachable destination). So an internet can have at most 15 routers between any two hosts. Note that the limit on network span is neither a limit on the total number of routers nor a limit on density. In fact, most campus networks have a small span even if they have many routers because the topology is arranged as a hierarchy. 52