WORKSHOPS

the gateway of last resort is not set. In the routing table for the Unix machine, the fourth entry defines the default route. If the machine receives a packet destined for a network that is not listed in the table, it will send the packet to the default gateway for delivery to the final destination.

The router has no equivalent gateway configured and informs us so. If the router receives a packet destined for a network number not listed in its routing table, it will not be able to forward t he packet.

The router also tells us how it learned of these destination networks. In the first column, "C" means that the network is directly connected to the router on the specified interface. "S" stands for static route and means that the route to this destination network is hard-coded into the router's configuration. "R" means that the router learned of this destination network via a RIP update. The rest of the entry in the router's routing table states which interface to use for the specified network and which router to send the packet to next, if appropriate.

The last entry in the Unix machine routing table requiring explanation is that for destination 127.0.0.1. This is a special reserved address kno wn as the loopback route for the local host. This address is used when the host needs to send datagrams to itself (as all hosts do).

How RIP Works RIP's job is to keep all routing tables in the network updated with the correct entries, by sending out regular routing-table-update messages.

R IP was designed for small homogeneous networks, where the best route is based on the number of hops (that is, the number of routers to go through) sufficient to maintain a fairly optimal routing table. In a network of this type, RIP works well and has a small overhead in terms of router-processor utilization.

Once configured on an internetwork, the operation of RIP and its interaction with the routing table is fairly straightforward. By default, every 30 seconds each RIP-enabled device sends out a RIP-update message, comprising routing information for every network the device knows about. The update includes the following: destination address of host or network, the first gateway along the route to this destination, the network interface that must be used to reach the first gateway, a metric that indicates the distance (in terms of hops) to the destination and the amount of time since the entry was last updated.

After a routing device receives an update, it processes the new information, which it co mpares with information in the existing routing table. If the routing update includes a new destination network, this information is added to the routing table. If the router receives a route with a smaller metric to a destination, it replaces the existing route. If an entry in the update message has the same destination network and gateway but a different metric, it will use this new metric to update the routing table.

This covers how routers handle a static network using RIP; some additions to the protocol are in place to handle changes in topolo gy, such as a downed link. If only the preceding logic was coded into a router, it would not recover from a link down, because it "remembers" only the best route to any given destination. If the gateway, or link to that gateway, should fail, the routing table might never reflect the change. Thus far, the logic depended on a gateway notifying its neighbors if its metrics changed. However, if the gateway could no longer communicate, it could not notify its neighb ors of a change. To handle such situations, RIP employs timers: It sends out messages every 30 seconds by default, and the protocol assumes that if a gateway is not heard from within 180 seconds, it is no longer available. Once it determines that a route is unavailable, the router sends a special message that notifies its neighbors of the unavailable route.

Chris Lewis is a product support manager at ILX Systems in New York.