Upgrading and Repairing Networks (5th Edition)

So far we've discussed routers and what they do, but we have not discussed the protocols involved. There are basic kinds of protocols that you need to understand when it comes to routers: routable protocols and routing protocols. For a protocol to be routable, it must make some provision for identifying a network as well as the host that's on a network. If a router had to keep track of every host on the Internet, it would be impossible to build a machine large enough to store all the routing table information. Instead, routable protocols specify network addresses as well as addresses of computers on those networks. Thus, routers only need to store a much smaller routing table that tells them where to forward packets based on the network address. After the packet reaches its destination network, it is delivered to the intended host using the host portion of the address field. TCP/IP (discussed in Chapter 24), for example, sets aside a portion of the IP address to use as a network ID. Other routable protocols include NetWare's IPX/SPX, DECnet, and AppleTalk. Each of these protocols can be used to create diverse network segments that can be tied together using a router. Many routers also support multiprotocol stacks and can route more than just TCP/IP. Examples of nonroutable protocols include Digital's (now HP) Local Area Transport (LAT) and NetBIOS (although NetBIOS over TCP/IP, or NBT, can overcome this limitation).

A routing protocol, on the other hand, is a protocol that routers use to communicate routing information among themselves. These protocols involve exchanging information about new routes, or old routes that no longer work, as well as metrics that describe the route (that is, the speed and hop count of the route). Routes are the path that a packet takes on its journey to its final network destination. At the local network, the host portion of the protocol address will be used to determine which computer on the local network the packet is destined for.

Routing protocols exchange information between routers so that routers gradually build up a view of the networks that can be reached. This information is dynamic because systems and routers can always experience downtime. When this happens, routers exchange information and, if a new route exists that can still reach the destination, the router may change its routing entries. When multiple routes to a destination exist, depending on the protocol, certain metrics (such as the number of hops between the source and destination) are used to determine what route a packet should take.

Note

Many kinds of routing protocols are in use today, each of which has its merits and shortcomings. Some are used for small internetworks, whereas others function on the Internet core routers. The most popular of these routing protocols are discussed in further detail in Chapter 33, "Routing Protocols."