A RIP Request message may request either a full routing table or information on specific routes only. In
the former case, the Request message will have a single route entry in which the address family identifier
is set to zero, the address is all zeros (0.0.0.0), and the metric is 16. A device receiving such a request
responds by unicasting its full routing table to the requesting address, honoring such rules as split horizon
and boundary summarization (discussed in "Classful Routing: A Summarization at Boundary Routers,"
later in this chapter).
Some diagnostic processes may need to know information about a specific route or routes. In this case, a
Request message may be sent with entries specifying the addresses in question. A device receiving this request will process the entries one-by-one, building a Response message from the Request message. If
the device has an entry in its routing table corresponding to an address in the request, it will enter the
metric of its own route entry into the metric field. If not, the metric field will be set to 16. The response
will tell exactly what the router knows, with no consideration given to split horizon or boundary
summarization.
As noted previously, hosts may run RIP in silent mode. This approach allows them to keep their routing
tables up-to-date by listening to RIP updates from routers without having to send RIP Response messages
uselessly on the network. However, diagnostic processes may need to examine the routing table of these
silent hosts. Therefore, RFC 1058 specifies that if a silent host receives a request from a UDP port other
than the standard RIP port of 520, the host must send a response.