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Metric Values

Dynamic routing based on RIP chooses the best route to some destination host or network based on the number of ``hops'', that is, the gateways a datagram has to pass before reaching it. The shorter a route is, the better RIP rates it. Very long routes with 16 or more hops are regarded as unusable, and are discarded.

To use RIP to manage routing information internal to your local network, you have to run gated on all hosts. At boot time, gated checks for all active network interfaces. If there is more than one active interface (not counting the loopback interface), it assumes the host is switching packets between several networks, and will actively exchange and broadcast routing information. Otherwise, it will only passively receive any RIP updates and update the local routing table.

When broadcasting the information from the local routing table, gated computes the length of the route from the so-called metric value associated with the routing table entry. This metric value is set by the system administrator when configuring the route and should reflect the actual cost of using this route. Therefore, the metric of a route to a subnet the host is directly connected to should always be zero, while a route going through two gateways should have a metric of two. However, note that you don't have to bother about metrics when you don't use RIP or gated.

Andrew Anderson
Thu Mar 7 23:22:06 EST 1996