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Unformatted text preview: le. On a point-to-point link this request is sent to the other end. The request
is broadcast if the network supports it. The destination UDP port is 520 (the routing
daemon on the other router).
This request packet has a command of 1 but the address family is set to 0 and the
metric is set to 16. This is a special request that asks for a complete routing table from
the other end.
Request received. If the request is the special case we just mentioned, then the entire
routing table is sent to the requestor. Otherwise each entry in the request is processed:
if we have a route to the specified address, set the metric to our value, else set the
metric to 16. (A metric of 16 is a special value called "infinity" and means we don't
have a route to that destination.) The response is returned.
Response received. The response is validated and may update the routing table. New
entries can be added, existing entries can be modified, or existing entries can be
Regular routing updates. Every 30 seconds, all or part of the router's entire routing
table is sent to every neighbor router. The routing table is either broadcast (e.g., on an file:///D|/Documents%20and%20Settings/bigini/Docu.../homenet2run/tcpip/tcp-ip-illustrated/dynamic.htm (4 of 17) [12/09/2001 14.46.53] Chapter 10. Dynamic Routing Protocols q Ethernet) or sent to the other end of a point-to-point link.
Triggered updates. These occur whenever the metric for a route changes. The entire
routing table need not be sent - only those entries that have changed must be
transmitted. Each route has a timeout associated with it. If a system running RIP finds a route that has not
been updated for 3 minutes, that route's metric is set to infinity (16) and marked for deletion.
This means we have missed six of the 30-second updates from the router that advertised that
route. The deletion of the route from the local routing table is delayed for another 60 seconds
to ensure the invalidation is propagated.
The metrics used by RIP a...
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- Spring '12