How to play for keeps

Tips for sizing up and choosing a Gigabit Ethernet backbone switch you'll want to grow old with.

By Edwin Mier
Network World, 1/26/98

Just as you're settling into a comfortable relationship with Fast Ethernet, Gigabit Ethernet sidles up to you. Gigabit Ethernet makes an attractive and reliable mate for your backbone, but be sure to take the time to really get to know a switch before choosing it as your long-term partner.

Although the ink on the relevant IEEE Gigabit Ethernet specifications is still drying, the switches are good to go. At last count, more than a dozen vendors were already shipping, or expected to soon begin shipping, Gigabit Ethernet backbone switches.

As lingering concerns about Gigabit Ethernet performance and interoperability evaporate, so do the risks associated with buying into the technology. And there's more good news - per-port prices for Gigabit Ethernet switches are poised to drop dramatically in response to growing supply and demand.

The downside is that Gigabit Ethernet switch vendors are churning out more marketing bravado and dubious one-up specmanship, making it difficult for you to pinpoint which product best meets your needs. Be sure to ask vendors for independent verification of switch performance and throughput.

You also should reserve some skepticism for product availability claims. Several vendors featured in the Buyer's Guide chart still were completing final testing on their switches at press time. If you suspect availability problems, don't hesitate to ask the vendor for customer references and check them.

If the products are ready and waiting, when should you buy? It largely depends on your network's requirements and budget. Keep in mind that today's first-generation Gigabit Ethernet switches are still fairly expensive at $2,000 to $3,000 per port.

Naturally, you'll get a better per-port cost the longer you wait. If Gigabit Ethernet prices drop the way Fast Ethernet did, you can expect Gigabit Ethernet switch ports to cost as little as $200 to $300 before the millennium. It's also reasonable to expect that any Gigabit Ethernet switch you buy six months to a year from now will support more features and options than today's models.

In addition to price, you should base your purchasing decision on these key factors: switch architecture and how it affects performance and scalability; standards support; and features such as concurrent routing, virtual LANs and trunking.

Look into backplane bandwidth

When you're evaluating products, keep in mind that not all switches are created equal. Some switch architectures may prevent you from using all of the backplane bandwidth capacity.

Because Gigabit Ethernet is only deployed in full-duplex mode to date, every Gigabit Ethernet port could be delivering 1G bit/sec of traffic to the switching fabric at the same time it is receiving 1G bit/sec.

Some Gigabit Ethernet switches, such as those from Bay Networks, Inc. and Prominet Corp., have a directional aspect to them. These switches' backplane bandwidths represent the total capacity in both directions. However, you wouldn't be able to fully utilize this capacity unless traffic were moving to and from the switching fabric in equal amounts, which is an unusual occurrence.

You ideally should look for at least 1G bit/sec of backplane bandwidth for each port in the maximum available configuration. More is good because it leaves you room to grow. But if there's less, you risk losing data with a fully configured switch under continuous heavy load.

However, you also need to look beyond the surface. In the case of Extreme Networks' Summit1 switch, for example, it would appear that the maximum configuration of 32 Gigabit Ethernet ports would greatly overwhelm the 17.5G-bit/sec backplane. But the maximum configuration is based on interconnecting four eight-port switches that each have a 17.5G-bit/sec backplane. The hardware required to link the switches is scheduled to ship this quarter.

Another major difference in the high-end switches is their support for other LAN types. Most of the switches in the chart support 10/100 Ethernet along with or in lieu of Gigabit Ethernet ports. Common packet sizes and formats make this a fairly clean mixture. But several Gigabit Ethernet switches also support ATM, and in a few cases, FDDI.

Moving data between these different high-speed LAN technologies is significantly more complex. If you're considering concurrently operating and interconnecting Gigabit Ethernet with ATM or FDDI, make sure you understand the throughput and latency implications.

Search for standards support

However, the Gigabit Ethernet specification is warranted to run only up to 260 meters over the type of fiber that's installed in most sites. Most vendors will guarantee that their products work up to 260 meters, and a few are willing to extend that warranted distance to about 500 meters.

The distance limitation could stifle Gigabit Ethernet's use in campuswide applications for now. Subsequently, many vendors intend to support a second flavor of Gigabit Ethernet called 1000Base-LX. This specification addresses the distance limitation because it is designed to run over single-mode fiber, which supports maximum distances of 3 to 10 km. Very few 1000Base-LX options were shipping as of late last year, though several are due for release this quarter. Most vendors' 1000Base-LX switch implementations are brand-new and may not have been thoroughly tested yet for distance or interoperability. And in a year or so, you can expect to see a standard for running Gigabit Ethernet over unshielded twisted pair cable.

The Gigabit Ethernet standards committee is reportedly still finalizing a few other issues, such as some aspects of autonegotiation and flow control. These don't necessarily pose interoperability problems, however. For example, one standard that is still incomplete will define how switches autonegotiate between half- and full-duplex modes at connection setup. But because everyone is only implementing full-duplex Gigabit Ethernet at this time, negotiating for half-duplex is a moot point.

Unlike its older Ethernet siblings, Gigabit Ethernet arrived at the same time as new standards for virtual LAN operation and setup. Look for a switch that supports the IEEE 802.1q standard, which spells out how to denote VLANs and how to tag packets. Many switch vendors were still implementing standards-based VLANs late last year, and no company that we contacted has performed any VLAN interoperability testing yet. However, the Buyer's Guide chart shows that the VLAN features of different Gigabit Ethernet switches are fairly consistent. All support VLANs based on port groupings. All but a few also support VLANs based on MAC addresses, as well as VLANs based on network-layer protocols.

Also investigate each device's routing support. Most Gigabit Ethernet switch vendors claim their products can function as high-capacity IP routers, though many are still working on implementing and delivering this capability.

Gigabit Ethernet switch vendors clearly differ in their IP routing implementations and IP protocol support, as evident in the accompanying chart. Their routing throughput rates also vary considerably, though many claim their devices can route IP at the same speed and rate at which they switch.

If you evaluate all of these factors before taking the plunge, you'll increase your chances of forming a Gigabit Ethernet relationship that works for the long haul.