Modular switch backplane performance: Know where to look

If you're considering buying a modular switch based upon rated backplane bandwidth, ask your vendor just how and where that bandwidth was measured. Today's modular switches provide as many as three different backplanes and the bandwidth of each may be different.

First, many switches provide Application Specific Integrated Circuit (ASIC) switching on each module. The port-to-port backplane is really just across the circuitry of a given module. Usually, traffic forwarded within a single module benefits from the best possible performance because the ceiling of switching performance is limited only by very powerful ASICs that quite often exceed the bandwidth of the attached LANs.

Second, modular switches also provide a chassis backplane that connects ports on different modules. For many products, the chassis backplane is engineered to support the simultaneous wire-speed operation of all ports in a full mesh configuration. However, this is not always the most cost-effective design. After all, you may end up paying a premium for performance that you'll never need. That is, if your network is even approaching 100% utilization for several seconds at a time, you should be planning an upgrade anyway. If that's the case, you may not need full wire speed on all ports all the time.

Third, many modular switches offer an expansion bus consisting of either a proprietary cable or a high-speed LAN uplink. In either case, you'll want to ask your vendor about the bandwidth of the uplink and any performance issues associated with crossing the uplink vs. crossing the chassis backplane. For instance, an ATM 622 uplink may be insufficient for a high-density Fast Ethernet switch.

In the end, before purchasing a modular switch, consider these important issues: Can users within one logical network group of your network be colocated on a single module to optimize performance, or must they cross the chassis backplane or uplink? How many ports will you need running at wire speed at a given time? Do you really need a switch that delivers wire speed on all ports or would, say, 80% be acceptable? If the switch provides an uplink, is it proprietary or a standards-based topology (e.g., ATM) and how does that affect your network strategy?

Many vendors talk about backplane bandwidth as just a flat throughput measurement. However, as we've seen, there are really three different backplanes that require careful consideration. Choosing a switch without understanding these issues may result in a network that performs poorly or that costs far too much.

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