This chapter began with an introduction to routing, including a look at the difference between routed (IP and IPX) and routing (RIP and IGRP) protocols. The communication process on a simple one-router network was described step-by-step, and included an overview of the different processes that take place when a host on one network attempts to communicate with a host on another. A more complex example, involving multiple routers and networks demonstrated the way in which routers will forward packets to the next-hop address on their journey between a source and destination host on different networks.
A look at static routing provided an overview of how this technique allows an administrator to manually define the path that a router will choose in reaching a network. This included a look at the IP routing table, and the syntax of the ip route command. The concept of administrative distances was also introduced, in order to illustrate how a router makes decisions on which routing table entry to use when a path to the same network is learned in different ways. For example, a static routing table entry will always be considered more trustworthy than a route learned from a routing protocol by default.
A look at dynamic routing demonstrated how routers are capable of exchanging routing table information with one another, both through the use of distance vector and link state routing protocols. The major differences between both types of routing protocols were outlined, as was the concept of a hybrid protocol. The danger of routing loops was also discussed, including a look at the techniques used by distance vector protocols to avoid these loops.
An overview of the distance-vector Routing Information Protocol outlined its major characteristics, timer values, and finally the commands to configure and monitor RIP on a Cisco router. The Interior Gateway Routing Protocol was also discussed, including an overview of its configuration. Default routing was looked at as a way to configure a gateway of last resort, the next-hop router to whom a router will send packets destined for unknown networks.
IPX routing was looked at next, including the fact that IPX RIP and SAP messages are broadcast between routers automatically after issuing the ipx routing command. The IPX routing table was looked at, as were the commands to monitor IPX RIP and SAP broadcast traffic. The show protocols command outlined a quick an effective way to obtain information on a router’s interfaces, including their current state and configured addresses.
