A recent trend in tying together network resources is to interconnect the various network segments using switches rather than hubs or routers. The easiest way to improve the performance of shared LANs is to add port or segment switching in which LAN segments are assigned to new ports instantly, allowing bottlenecks to be eliminated through the reassignment of very active LAN nodes.
For example, several dozen workstations running network bandwidth-intensive applications such as imaging, video editing, and computer-aided design on the same Ethernet segment can produce a serious bottleneck. LAN switching can segment an overcrowded, shared-bandwidth workgroup into multiple virtual LANs in which each user or group of users can access 10-Mbps Ethernet or 16-Mbps token-ring bandwidth. The solution is cost-effective and improves network performance for each user.
Several types of switches are now available for Ethernet LANs. In addition to the basic 10BaseT switch, there are auto-sensing 10/100-Mbps switches and Gigabit Ethernet switches. There are also switches for Token Ring. However, the hottest trend is the use of Layer 3 switches, which improve the performance of segmented Ethernet and Token Ring LANs without having to use routers. Aside from cost savings, the benefits are improved availability and throughput. Sometimes Layer 3 capabilities come with or can be added to the other types of switches.
Since existing investments in network topology, cabling, adapter cards, and operating systems are retained, the costs associated with implementing a switched LAN solution come mainly from the purchase of the new switch itself and any additional management tools that may be required.
Layer 3 Switches
Layer 3 switches deserve some explanation because they are not really switches. With all the pressure to differentiate their products, some vendors confuse buyers by applying the term to products that range from bridges to routers. Like ordinary LAN switches, Layer 3 switches make forwarding decisions based on the packet’s destination address. But they also have the intelligence to make decisions based on information stored inside the packet, just like routers. So these “Layer 3 switches” really are not switches at all, but routers with faster frame-forwarding capabilities.
One reason why Layer 3 devices operate so fast is that they usually support only IP. When other protocols are supported, such as Novell’s IPX, Apple’s AppleTalk, or IBM’s APPN, the packet forwarding rate is the same as traditional routers or even slower. When added to the network, Layer 3 devices can make routers more efficient by off-loading the IP routing task. Since IP is the most often used protocol, off-loading IP from the routers can provide a fairly substantial performance gain.
Adding the Layer 3 capability to a network does not always require buying a whole new system. Some vendors, such as Cisco, offer add-ons to their existing LAN switches that transform them into Layer 3 systems.
Workgroup Switches
Although there are feature-rich enterprise-level switches, most LAN switches are implemented at the department or workgroup level. Prices, performance, and features vary widely. Some devices are basic, no-frills switches, while others come with virtual LAN support, protocol filtering, SNMP management, high-speed uplinks, and remote monitoring (RMON) support.
For example, 3Com’s SuperStack II Switch 1100 is priced at $65 per port. It has 12 10BaseT ports and features two built-in auto-sensing 10/100 Fast Ethernet ports. A matrix port enables high-speed connection to other Su-perStack II units. The stacked switches can be managed as a single entity and share a single IP address. The switch automatically provides full-duplex/half-duplex capability on all ports to boost bandwidth for servers and power users. It also implements advanced policy-based management across the network and supports Fast IP, IGMP snooping, IEEE 802.1p prioritization, and IEEE 802.1q standards-based VLANs. The company offers a 24-port unit priced at about $100 per port.
There are 10/100BaseT switches that fall into the same price range as 10BaseT switches. D-Link Systems, for example, offers a 24-port, 10/100-Mbps, dual-speed switch priced at $80 per port. The Layer 2 switch has 5.5G bps of total bandwidth, supports SNMP and RMON, and is targeted for workgroup or departmental deployments.
Usually, Token Ring networks carry a higher price tag than Ethernet. Although prices for Token Ring workgroup switches are dropping, the lowest price of $169 per port from Madge Networks is still twice the per-port price of an Ethernet switch.
In the 12- to 24-port range, 100BaseT switches tend to be stackable, allowing the system to scale to as many as eight units. Many are of fixed configuration and do not allow for the addition of more ports, except by adding an entire system. When designing switched networks, using systems of differing port capacities may be appropriate. However, two eight-port switches do not necessarily equate to one 16-port switch. With two switches, there is usually a bigger bottleneck when packets must travel between ports on different switches.
For network managers unsure of which technology their organizations might migrate to in the future—ATM or Gigabit Ethernet—dual-speed Ethernet stackable switches now offer high-speed uplink modules for ATM or Gigabit Ethernet backbone connections. This kind of switch is a risk-free buy for companies not sure which high-speed LAN technology they will choose but who will likely need to upgrade to ATM or Gigabit Ethernet in the future.
Management Tools
Although current shared-LAN tools handle visibility of the network and the bandwidth usage of hub segments in a switched environment, visibility of the switched ports is lost. This leaves network managers unenlightened about the source of the network traffic. The difference in managing a shared versus a switched network is that in a shared environment, all the tools and agents assume a single, shared interface into the hub. In a switched environment, every port behaves as an individual LAN. This behavior requires an individual interface to every port, in addition to an interface into the box.
External network management tools such as LAN analyzers, port probes, and monitors have become standard equipment for network managers. But they can be expensive; the hardware alone could cost between $3,000 and $5,000, and the RMON agent software could cost as much as $10,000. In addition, a comprehensive data collection and analysis tool such as a network sniffer costs about $25,000. Alternatively, monitoring only one LAN segment at a time—on a 24-port switch, for example—requires a lot of time and effort to gather all the necessary information.
Some vendors use the switch’s internal capability to create a roving RMON probe that allows statistics to be gathered on port segments at specified intervals. Management information can be gathered from multiple ports with a single probe without having to incur the expense of putting RMON on every port. Although the roving RMON probe is a simpler and more economical solution than the manual network sniffer, it still does not provide a global view of the network.
Some vendors offer enhanced network configuration and management features, which are aimed at LAN managers scrambling to optimize growing networks. Bay Networks, for example, offers enhancements for its BayStack Ethernet switches that include the ability to configure port-based VLANs, port mirroring, Web-based management, and unlimited media access control (MAC) addressing. Network management support for these switches is provided through integration with Bay Networks’ Op-tivity network management software. Other vendors, including 3Com, offer many of the same management features, plus broadcast traffic control, RMON on every port, and policy-defined management.
Web-based management tools facilitate installation, configuration, control, and troubleshooting of network switches. Using a familiar Web interface, such tools enable LAN managers to administer switches from any networked PC. The interface presents graphical information the LAN administrator can interpret quickly instead of having to read lines of data. Using a Web browser instead of a dedicated management console or a terminal at the switch itself saves a LAN manager considerable time and support costs. Many vendors offer Web-based management tools free, even making them available for download from their Web site.
Last Word
LAN switches are available in high-end, multislot matrix systems, which typically provide aggregate switching capacities in excess of 1 Gbps and support multiple high-speed technologies besides 100BaseT. These switches, usually with built-in Layer 3 capability, can accommodate virtually any type of LAN interface. Vendors offer modules that can interconnect 100BaseT, 10BaseT, 100VG-AnyLAN, FDDI, and ATM networks.
There is a wide variety in the price and throughput performance of LAN switches of all kinds. The performance claims of vendors do not always hold up under rigorous testing by independent sources. Given the disparity in claimed versus actual performance—and all other things being equal, such as management and flow- and traffic-control features— the high prices charged by some vendors needs to be negotiated down before committing to a purchase, or the buyer needs to look elsewhere.
