Chapter 3 will discuss the design implications of the differences between routing and bridging. The discussion of the OSI model here makes it a good place to define them and talk about their technical differences.
I will use the terms "bridging" and "switching" interchangeably throughout this book. This is because early manufacturers of multiport fast bridges wanted to make it clear that their products were distinct from earlier products. The earlier products, called "bridges," were used primarily for isolation and repeating functions; the newer products tended to focus on reducing latency and increasing throughput across a network. Technically, they perform the same basic network functions. But these vendors wanted to make sure that consumers understood that their products were different from the earlier devices: so they gave them a different name.
To make matters more confusing, it has become fashionable to talk about "Layer 3 switches." These are essentially just routers. But, in general, they are special-function routers that route between like media, which allows certain speed optimizations. So, where you might use a Layer 3 switch to route between two VLANs, both built on Fast Ethernet, you would never use one to control access to a WAN. You probably would want to think very carefully before using a Layer 3 switch to regulate traffic between a Token Ring and an Ethernet.
Routing means sending packets from one Layer 3 network region to another using Layer 3 addressing information. These two Layer 3 regions could use different Layer 1 or 2 protocols. For example, one may be Ethernet and the other ATM. So part of the routing process requires taking the Layer 3 packet out of the Ethernet frame in which it was received, deciding where to send it, then creating ATM cells to carry this packet. Because ATM uses a cell size that is much smaller than the Ethernet packet size, the router has to chop up the Layer 3 packet and wrap each fragment in an ATM cell before sending it. When receiving from the ATM side, it has to wait until it receives all of the ATM cells that form one Layer 3 packet, reassemble the fragments in the correct order, and wrap it up in an Ethernet frame before sending it on. This allows easy transfer of data between LAN and WAN or between different LAN types.
Technically, bridging has some overlap into the Network Layer as well, because it specifies how the broadcast domains that are part of the Data Link Layer can interact with one another. But the special difference between routing and bridging is that in routing the Data Link properties don't need to have anything in common. It is easy to route IP from Ethernet to Token Ring without needing to consider anything but the IP addresses of both ends. But in bridging, the MAC (Media Access Control) addresses from one side of the bridge are maintained as the frame crosses over to the other side.
It is possible to bridge from Ethernet to Token Ring, for example. But the Token Ring devices must believe that they are talking to another Token Ring device. So the bridge has to generate a fake Token Ring MAC address for each Ethernet device, and a fake Ethernet MAC address for each Token Ring device taking part in the bridge.
With routing, though, there is only one MAC address visible, that of the router itself. Each device knows that it has to communicate with all off-segment devices through that address.
So routing scales much better than bridging when large numbers of devices need to communicate with one another. But the drawback is that the extra work of translating from one data-link layer to another means that the router has to read in every packet, decide where to send it, reformat it for the new medium, and then send it along.
With switching, however, it is possible to read in just enough of the packet to figure out where it needs to go and then start sending it out before it has all been received. This is called cut-through switching. Store-and-forward switching, in which the entire packet is read before forwarding, is also common. But the bottom line is that switching is generally faster than routing.
Layer 3 switching is sort of a hybrid. If you know that you are switching between like media, then the only things you need to change when you pass the packet along are the source and destination MAC addresses (and the checksum will also need to be corrected). This is considerably less work than the general media-independent problem of routing. So these Layer 3 switches are usually faster than a general-purpose router.
The other advantage of a Layer 3 switch over a router is that it can often be implemented as a special card in a Layer 2 switch. This means that it is able to do its work while touching only the backplane of the switch. Because the switch backplane doesn't need to go very far, and because it usually runs a proprietary high-speed protocol, it is able to run at extremely high speeds. So it is as if you were able to connect your router, not to a 100-Mbps Fast Ethernet or even to 1000Mbps Gigabit Ethernet, but to a medium many times faster than the fastest readily available LAN technologies. And this is done without having to pay a lot of extra money for the high speed access.
Chapter 3 will discuss how to use these sorts of devices effectively.