24-port NWay Ethernet Switch User’s Guide
Switch Management and Operating Concepts
125
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 infinigy, 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. Consider, for
example, a typical corporate intranet. Most use a hierarchy that
consists of a high-speed backbone network with multiple routers
each connecting the backbone to a workgroup, where each
workgroup occupies a single LAN. Although the corporation can
include dozens of workgroups, the span of the entire intranet is
only 2. Even if each workgroup is extended to include a router
that connects one or more additional LANs, the maximum span
only increases to 4. Similarly, extending the hierarchy one more
