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.

Characteristics

Layer 2 switches themselves act as IP end nodes for Simple Network Management Protocol (SNMP) management, Telnet, and Web based management. Such management functionality involves the presence of an IP stack on the router along with User Datagram Protocol (UDP), Transmission Control Protocol (TCP), Telnet, and SNMP functions. The switches themselves have a MAC address so that they can be addressed as a Layer 2 end node while also providing transparent switch functions. Layer 2 switching does not, in general, involve changing the MAC frame. However, there are situations when switches change the MAC frame. The IEEE 802.1Q Committee is working on a VLAN standard that involves ?tagging? a MAC frame with the VLAN it belongs to; this tagging process involves changing the MAC frame. Bridging technology also involves the Spanning-Tree Protocol. This is required in a multibridge network to avoid loops.

The same principles also apply towards Layer 2 switches, and most commercial Layer 2 switches support the Spanning-Tree Protocol. The previous discussion provides an outline of Layer 2 switching func-tions. Layer 2 switching is MAC frame based, does not involve altering the MAC frame, in general, and provides transparent switching in par-allel with MAC frames. Since these switches operate at Layer 2, they are protocol independent. However, Layer 2 switching does not scale well because of broadcasts. Although VLANs alleviate this problem to some extent, there is definitely a need for machines on different VLANs to communicate. One example is the situation where an orga-nization has multiple intranet servers on separate subnets (and hence VLANs), causing a lot of intersubnet traffic. In such cases, use of a router is unavoidable; Layer 3 switches enter at this point.

Layer 3 Switches

Layer 3 switching is a relatively new term, which has been ?extended? by a numerous vendors to describe their products. For example, one school uses this term to describe fast IP routing via hardware, while another school uses it to describe Multi Protocol Over ATM (MPOA). For the purpose of this discussion, Layer 3 switches are superfast rout-ers that do Layer 3 forwarding in hardware. In this article, we will mainly discuss Layer 3 switching in the context of fast IP routing, with a brief discussion of the other areas of application.