Lan Switches

Layer 2 switches are frequently installed in the enterprise for high-speed connectivity between end stations at the data link layer. Layer 3 switches are a relatively new phenomenon, made popular by (among others) the trade press. This article details some of the issues in the evolution of Layer 2 and Layer 3 switches. We hypothesize that that the technology is evolutionary and has its origins in earlier products.

Layer 2 Switches

Bridging technology has been around since the 1980s (and maybe even earlier). Bridging involves segmentation of local-area networks (LANs) at the Layer 2 level. A multiport bridge typically learns about the Media Access Control (MAC) addresses on each of its ports and transparently passes MAC frames destined to those ports. These bridges also ensure that frames destined for MAC addresses that lie on the same port as the originating station are not forwarded to the other ports. For the sake of this discussion, we consider only Ethernet LANs.

Layer 2 switches effectively provide the same functionality. They are similar to multiport bridges in that they learn and forward frames on each port. The major difference is the involvement of hardware that ensures that multiple switching paths inside the switch can be active at the same time. For example, consider Figure 1, which details a four-port switch with stations A on port 1, B on port 2, C on port 3 and D on port 4. Assume that A desires to communicate with B, and C desires to communicate with D. In a single CPU bridge, this forwarding would typically be done in software, where the CPU would pick up frames from each of the ports sequentially and forward them to appropriate output ports. This process is highly inefficient in a scenario like the one indicated previously, where the traffic between A and B has no relation to the traffic between C and D.

Figure 1: Layer 2 switch with External Router for Inter-VLAN traffic and connecting to the Internet

Enter hardware-based Layer 2...