Over the preceding few chapters, we've covered the gamut of network devices. Switches sit in a data closet, taking twisted-pair cables from host devices, forming the LAN, and they also pack the power to support VLANs. Another inhabitant of the data closet is the access server, used to link remote users into internetworks through Internet VPNs or dial-in telephone lines. If a packet moves beyond its source LAN segment, it flows onto a backbone LAN, where it encounters a router and (if it's a secured internetwork) a firewall. After that point is the Great Beyond. Once the packet goes past the local network, it enters a realm of seemingly infinite complexity.
Internetworks are complex because they're big and subject to endless fluctuation. An internetwork's topology is altered whenever a new switch is added or when a router is inserted to help direct growing internetwork loads. As usage patterns evolve, traffic congestion seems to pop up in different spots every day. If network devices crash, they take their connected LAN segments down with them, and traffic must be immediately redirected-and then redirected back once the downed device is brought back online. More frequently, the network device is up but one of its network interfaces has gone down, or the interface is okay, but a cable was accidentally knocked from its port. To top it all off, sometimes all the physical network equipment is running fine, but things still go awry because a rotten config file was somehow introduced into the mix!
The point here is that large internetworks are simply too complicated to be managed by people alone. Imagine a roomful of network administrators trying to manually control each and every network event in a Fortune 500 company, and you'd see a portrait of creeping disorganization. Now imagine that same room-or even a building-filled with people attempting to corral the Internet itself, and you see unmitigated chaos. There's just too much complexity and change to handle without a constant source of reliable help-automated help.
So how does it all work? How do packets find their way across internetworks with the reliability we've come to take for granted? The answer is routing protocols.