TCP IP Illustrated

The request is broadcast if the network supports it

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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 deleted. 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. Metrics The metrics used by RIP a...
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