IP Routing

Underlying Processes

Behind the scenes, there are three key sets of processes running on each router that make up IP routing. I have already discussed examples from each of these three sets in the preceding sections. These processes may be organized into three categories:

1. Processes associated with the discovery of paths to various destinations in the network. These processes include dynamic routing protocols, such as RIP and IGRP, as well as static route entries. This text describes these processes in detail.

2. Processes that maintain the IP routing table. These processes receive updates from all dynamic routing protocols running on the router as well as from static route entries. By attaching administrative distance values to each routing information source, these processes break ties when multiple sources (e.g., OSPF and static route entries) report paths to the same destination. I discussed the use of administrative distance values in the previous section. Other examples from this group of processes will be discussed in Chapter 8.

3. Processes involved with the forwarding of IP packets. These processes are invoked when a router receives a packet to forward. The result of the match between the destination IP address in the packet and the contents of the IP routing table may be a match with one entry in the routing table, a match with more than one entry in the routing table, a match with a default route, etc. One general rule here is the rule of longest prefix match -- if there is more than one match, the match with the longest subnet mask (or prefix) wins. Further, the outcome of these processes depends on whether the router is configured for classful or classless route lookups.

Several concepts that have not yet been discussed were thrown into the preceding discussion. For instance, we have not yet talked about classful versus classless route lookups or about default routes. These concepts will be addressed in later chapters. However, this early lesson in the division of processes should help you to understand and classify concepts more quickly.