A routing table has columns for at three types o information the network ID the cost and the ID of the next router. The network ID is the final destination of the packet. The cost is the number of hops a packets must make to get there. And the next router is the router to which a packet must be delivered on its way to a particular destination.
The original routing tables for our sample internetwork are show in this point the third column is empty because the only destination network identified are those attached to the current router.
No multiple hop destination and therefore no next router have been identified. These basic tables are sent out to neighbours. For example A sends is routing take to routers B, F sends E, B send it routing table to routers C and A and so on:
When a receives a routing from B it uses the information to update its own table. A adjusts the information shown B table by adding one to each listed cost. It then combines the table with it own to create a new more comprehensive table.
This process continues for all routers. Every router receives information from neighbours and updates its routing table. If there are no more changes the final tables may look like those shown.