D-Link DGS-3308 Series, Руководство пользователя

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8-port Gigabit Ethernet Switch User’s Guide
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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.