Today the division between routers and switches is a fine line. Whereas switches were initially designed to help segment a LAN into multiple collision domains, and thereby allow you to extend the reach of a particular LAN topology, switches have moved higher up the networking ladder. When switching is used in a LAN to connect individual client and server computers, the process is known as microsegmentation, because the collision domain has been reduced to just the switch and the computer attached to a port. Switches at this level generally work using the hardware (MAC) addresses of the attached computers.
Layer 3 switching moves switching up the ladder by one rung by switching network frames based on the OSI Network layer address—an IP address, for example. But wait, that's what a router does, isn't it? Of course. A layer 3 switch is basically a router, but it implements most of its functions in application-specific integrated chips (ASICS) and performs its packet processing much faster than does a traditional router, which uses a microprocessor (much like a computer CPU) for this function.
When you get to the top of the ladder, where large volumes of data need to be routed through a large corporate network—or the Internet, for that matter—even the fastest traditional routers or layer 3 switches easily can become bogged down by the volume of traffic. Because of this, the core of a large network traditionally has been built using ATM or Frame Relay switches, and IP traffic is sent over these switched networks.
To speed up the processing of routing packets at high-volume rates, a newer technology has been developing over the past few years and goes by the name of Multi-Protocol Label Switching (MPLS). MPLS is covered in the next section.
Combining Routing and Switching
Traditional routers have a large amount of overhead processing they must perform to get a packet to its destination. Each router along the packet's path must open up and examine the layer 3 header information before it can decide on which port to output the packet to send it to its next hop on its journey. If a packet passes through more than just a few routers, that's a lot of processing time. Remember that IP is a connectionless protocol. Decisions must be made about a packet's travel plans at each stage of its journey through the network. The solution to this problem lies in newer technology—high-speed switching. Specifically, Multi-Protocol Label Switching, which is discussed in the next section, combines the best of routing techniques with switching techniques.
When you look at concepts such as ATM or Frame Relay, which are connection-oriented protocols, this isn't the case. Instead, virtual circuits (either permanent or switched) are set up to connect to endpoints of a communication path so that all cells (as in the case of ATM) or frames (as in the case of Frame Relay) usually take the same path through the switched network.
