This layer, known as the switching layer, allows end station addressing and attachment. Because architectures up to Layer 2 allow end station connectivity, it is often practical to construct a Layer 2-only network, providing simple, inexpensive, high-performance connectivity for hundreds or even thousands of end stations. The past five years have seen the extraordinary success of the “flat” network topologies provided by Layer 2 switches connected to other Layer 2 switches or ATM switches.
Layer 2 switching, also called bridging, forwards packets based on the unique Media Access Control (MAC) address of each end station. Data packets consist of both infrastructure content, such as MAC addresses and other information, and end-user content. At Layer 2, generally no modification is required to packet infrastructure content when going between like Layer 1 interfaces, like Ethernet to Fast Ethernet. However, minor changes to infrastructure content?not end-user data content?may occur when bridging between unlike types such as FDDI and Ethernet. Either way, processing impact is minimal and so is configuration complexity.
Layer 2 deployment has seen the most striking infrastructure change over the past decade. Shared Ethernet, represented by particular cable types or contained within shared hubs, offered a very simple, and even more inexpensive, approach for Layer 2. Though still quite popular, shared technology, where all stations use the same bandwidth slice, has very limited scaling capabilities. Depending upon the applications being used, shared networks of more than one hundred users are becoming less common. Many network designers have “tiered” their infrastructure by feeding shared Layer 2 into switched Layer 2 or even Layer 3. Switched Layer 3 apportions each station?or port?its own dedicated bandwidth segment. Recent enhancements at Layer 2 provide packet prioritization capabilities for the application of network policies. The new IEEE 802.1p standard defines Class of Service (CoS) policies capabilities for Layer 2 segments.
Note that Layer 2 does not ordinarily extend beyond the corporate boundary. To connect to the Internet usually requires a router; in other words, scaling a Layer 2 network requires Layer 3 capabilities.