Bridges, Switches, and Routers

Bridges, Switches, and Routers

It is easy to become confused about the distinction between bridges, switches, and routers. There is good reason for such confusion, since at some level, they all forward messages from one link to another. One distinction people make is based on layering: Bridges are linklevel nodes (they forward frames from one link to another to implement an extended LAN), switches are network-level nodes (they forward packets from one link to another to implement a packetswitched network), and routers are internet-level nodes (they forward datagrams from one network to another to implement an internet). In some sense, however, this is an artificial distinction. It is certainly the case that networking companies do not ask the layering police for permission to sell new products that do not fit neatly into one layer or another.

For example, we have already seen that a multiport bridge is usually called an Ethernet switch or LAN switch. Thus the distinction between bridges and switches has now been largely eroded. For this reason, bridges and switches are often grouped together as “layer 2 devices,” where layer 2 in this context means “above the physical layer, below the internet layer.”

There is, however, an important distinction between LAN switches (or bridges) and ATM switches (and other switches thatare used in WANs, such as Frame Relay and X.25 switches). LAN switches and bridges depend on the spanning tree algorithm, while WAN switches generally run routing protocols that allow each switch to learn the topology of the whole network. This is an important distinction because knowing the whole network topology allows the switches to discriminate among different routes, while in contrast, the spanning tree algorithm locks in a single tree over which messages are forwarded. It is also the case that the spanning tree approach does not scale as well.

What about switches and routers? Are they fundamentallythe same thing, or are they different in some important way? Here, the distinction is much less clear. For starters, since a single point-topoint link is itself a legitimate network, a router can be used to connect a set of such links. In such a situation, a router looks just like a switch. It just happens to be a switch that forwards IP packets using a datagram forwarding model and IP routing protocols.

One big difference between an ATM network built from switches and the Internet built from routers is that the Internet is able to accommodate heterogeneity, whereas ATM consists of homogeneous links. This support for heterogeneity is one of the key reasons why the Internet is so widely deployed.