Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 98
A Bigger Update

This slide shows what happens when router A submits its routing table out of its port connected to network 2. This slide assumes all routers are newly initialized. (For simplicity, the slide shows the updating through one port only. In reality, routing tables are submitted out all ports of a router, with a few restrictions on which entries of the table get transmitted.)

Router A transmits its table containing two networks: Z and Y. Each of these networks is one hop away (they are directly connected). Router B will receive this packet and will add 1 to each hop-count entry in the received table. (This is accomplished assuming the RIP cost assigned to that port of router B is 1. The configured hop count could be set to something else.)

Router B examines its table and notices that it does not have an entry for network Z. It will add this entry to its table as: network 1, available through port 1, two hops away. It will then check the next entry. Network Y will not be added, for router B already has network 2 in its table with a cost of 1. Since the incoming table reports network Y has a cost of 2, router B will ignore this entry. (There are rules that will prevent router A from sending out information about network 2, which will be discussed later.)

Once its table is updated, router B will transmit its table out its ports every 30 seconds (again, for simplicity only one router update is being shown). Router C will receive this table from router B and will perform the same steps as router B. Eventually, all information about all networks on the internet will be propagated to all routers.

A Bigger Update