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1.7 The Structure of a Routing Table

Standard distance vector protocols broadcast the whole routing table to immediately connected routing devices in flash updates and regular update periods. Link-state protocols use triggered multicast advertisement packets to alert other routers on topology changes. The specific topology changes are transmitted, not the whole routing table. OSPF differs slightly; it transmits a declaration each 30 minutes in order to sustain the synchronization of the topology database between all routers in the vicinity. The IP routing table can be shown by keying show ip route at the IOS exec prompt. The IP routing table contains the information necessary to correctly execute the routing operations.

In addition to that information, the IP Routing Table also holds other relevant bits of information, some of which are listed below:

The AD number is contained in the routing table.

The destination network

The interface that is used to reach the destination network The method that route was learned about.

The metric as a hop count

The logical address of the next hop mechanism

The quantity of time that the data is in the routing table, measured from the last update time.

The IP Routing Table

The IP Routing table shows how the route was learned via the RIP routing protocol indicated by r and the destination network of 183.15.9.0/25. The administrative distance for the RIP in this instance is 121 with a hop count or metric of 1 as indicated by the [121/1]. The logical address of the next hop mechanism is indicated as 183.16.2.1. The entry has existed for 21 seconds from the time when the last update occurred. Interface seriaio/2 was use to reach the destination. By stipulating a single network number by means of the show ip route [network] command, more information can be obtained about a specific route in the routing table.