Layer 3 Gigabit Ethernet Switches

By J.B. MilesSpecial to GCNThe wait is over'Gigabit Ethernet has come into its own. It is now a strong technology with a set of mature standards that can deliver 1-Gbps speed across networks on both fiber-optic and Category 5 copper media, and it's beginning to outpace asynchronous transfer mode for high-bandwidth networking.During the second quarter of this year, Gigabit Ethernet outshipped ATM LAN products for the first time. The margin likely will widen dramatically in the months to come as vendors ramp up to provide new Gigabit 1000Base-T products compatible with in-house Category 5 copper wiring.This guide features Layer 3 Gigabit Ethernet switches intended for primary use as enterprise backbone switches and high-speed links for server farms. Units listed in the accompanying chart provide at least six, and usually more, 1-Gbps fiber-optic or copper-wire ports that may be configured in several ways. For example, you could have eight 1000Base-SX ports, or six 1000Base-SX and two 1000Base-LX ports, depending on the modules, or blades, that come with the switches.Right now, the majority of Gigabit Ethernet switch ports are either multimode fiber (SX) or single-mode fiber. There also is fiber for spanning long distances between or across campuses. But look for 1000Base-T switch configurations to come on strong in the months ahead.Some models listed are pure Gigabit Ethernet switches, configurable only with Gigabit Ethernet ports. Others may also be configured with modules containing 10/100-Mbps Ethernet ports, or even ATM modules. Typically, the aggregate capacity of each switch depends on the number and types of ports; 17.5 Gbps is common for an eight-port 1000Base-SX configuration.For this guide, I distinguished between 10/100-Mbps Ethernet switches with one or two Gigabit Ethernet uplinks and switches that are essentially Gigabit-class.'The 10/100 Ethernet-Mbps switches are often used to link server farms with network backbones and may provide one or two Gigabit Ethernet uplinks for connecting to high-speed backbones or other switches.But they still function primarily as 10/100 Mbps-switches. The units in this guide serve mainly as 1-Gbps Gigabit Ethernet switches, although some are actually multifunctional switches with 10/100-Mbps Ethernet or even ATM modules.Within the seven-layer Open Systems Interconnection network model, Gigabit Ethernet switches have established themselves at Layer 2 but are a new development at Layer 3.Because they bring 1-Gbps speeds to the network, Layer 2 Gigabit Ethernet switches have given network managers a powerful incentive to rethink their old networking paradigms. As bandwidth-intensive multimedia, video, and Internet and intranet applications have spread across the enterprise, users have begun the transition from 10/100-Mbps Ethernet to Gigabit Ethernet, especially at the enterprise backbone level.The main benefit of Layer 2 Gigabit Ethernet switches is raw speed, measured not only in megabits/sec but also in the numbers of packets, about 1.4 million per second, they can transmit.In accordance with the OSI model, Layer 2 switches operate at the second, or data-link, layer. Their main task is to check incoming traffic, keep track of the physical addresses of all stations attached to their ports and forward traffic to the specific addresses according to the forwarding tables they have built. Layer 2 switches are fast because they don't have to examine individual data packets, just pass them on. Thus, they can deliver data at wire speeds, or up to the limits of the physical connections themselves.But although Layer 2 switches are fast, they cannot handle the traffic bottlenecks caused by aging routers spread across the network. Conventional software routers must examine individual data packets before identifying their destinations and sending them on their way. The problem is that they can only handle several hundred thousand frames of data per second. This is far too slow for today's high-speed bandwidth requirements, as more servers, workstations and applications are added to the enterprise.Layer 3 switches, also called routing or IP switches, perform both switching and routing functions simultaneously. They work at the third, or network, layer of the OSI mode that contains the IP address.Layer 3 switches make use of hardware-based, app-specific integrated circuits and specialized shared-memory or cross-bar switching fabrics. This, along with their fast CPUs and other components, accounts for their wire-speed routing capability. Layer 3 switches can be used as cutting-edge replacements for legacy network routers.Another benefit of Layer 3 is an assured quality of service that Layer 2 switching doesn't provide alone. The newest Layer 3 switches provide simultaneous filtering for layers 2, 3, 4 and above that helps guarantee your mission-critical data will get where it has to go'all without slowing down the network. In the future, look for more Layer 4, or traffic-control, capability.Layer 3 Gigabit Ethernet switches represent a big network breakthrough; consequently there isn't a large selection of them available yet. Mainline switch manufacturers such as 3Com Corp., Cabletron Systems Inc., Hewlett-Packard Co., IBM Corp. and Nortel Networks Corp. are busy adding Gigabit Ethernet capability with Layer 3 and 4 management modules to their Layer 2 switches.Industry newcomers might have a marketing edge because they began life as Gigabit Ethernet manufacturers. XXXSPLITXXX-