All routing protocols follow the same patterns of operation. To help illustrate this, consider the following scenario in which all three routers are running RIPv2.
When a router powers up, it knows nothing about the network topology. It does not even know that there are devices on the other end of its links. The only information that a router has is from its own saved configuration file stored in NVRAM. After a router boots successfully, it applies the saved configuration. If the IP addressing is configured correctly, then the router initially discovers its own directly connected networks.
Click Play in the figure to view an animation of the initial discovery of connected networks for each router.
Notice how the routers proceed through the boot up process and then discovers any directly connected networks and subnet masks. This information is added to their routing tables as follows:
- R1 adds the 10.1.0.0 network available through interface FastEthernet 0/0 and 10.2.0.0 is available through interface Serial 0/0/0.
- R2 adds the 10.2.0.0 network available through interface Serial 0/0/0 and 10.3.0.0 is available through interface Serial 0/0/1.
- R3 adds the 10.3.0.0 network available through interface Serial 0/0/1 and 10.4.0.0 is available through interface FastEthernet 0/0.
With this initial information, the routers then proceed to find additional route sources for their routing tables.