Back to the review Interactive buyer's guide

By Edwin E. Mier
Network World, 8/11/97

Many users have already found that a little switching, in just the right places, can help a network run better. But will a lot of switching, deployed in every wiring closet, necessarily help the same network run a lot better?

According to market projections, most large user organizations will be spending a lot of money to find out. As a technology, switching has put older, shared-bandwidth hubs and repeaters out to pasture.

Switched Ethernet has proven itself. User attention is now focusing on the next echelon of higher capacity switches - being deployed in backbones, at least for now - which can switch multiple 100M bit/sec, even gigabit, datastreams.

There are several key areas, many of which are covered in the chart (beginning below), that you need to consider before you go switch shopping to make sure you bring home the right switch for your network. Key areas include:

1. Configuration options. These are as broad and diverse as the price range for 100Base-T switches. Do your switches need to support and interconnect just Ethernet and Fast Ethernet?

Most are now able to handle both of these with equal facility. Or do you also need to connect to an ATM, FDDI or Gigabit Ethernet network?

2. Throughput and performance. Most switches in this class offer ports that automatically sense between 10M Ethernet and 100M bit/sec Fast Ethernet. But can the same switch handle all ports at full 100M bit/sec port speed? An eight-port 10/100 switch could potentially receive, and need to switch, nearly 600,000 packet/sec. That's more than 300M bit/sec of data traffic. And this presumes all packets are unicast - addressed to a single destination. Broadcast packets add to the burden because each one has to be replicated by the switch and sent out onto each outbound port.

3. Switch-to-switch connectivity. Even at 100M bit/sec, your switch-to-switch 100Base-T links can easily become bottlenecks, not to mention single points of failure. Most switch vendors support full-duplex operation, in which an unshielded twisted-pair link can be set to handle up to 100M bit/sec in each direction. Some support multiple parallel switch-to-switch links, automatic switchover to a redundant link, even load balancing, which can ameliorate these concerns.

4. What's standard and what's not.

The answers affect how different vendors' switches interoperate or should interoperate - and how they don't or won't.

One of the nicest things about switches is that they require minimal setup or configuration - especially when compared with routers. Still, every switch is designed with a specific load and number and type of ports in mind.

It will cost appreciably more to buy a switch that's a fully modular, chassis-based system, especially if it can handle switching between different high-speed LAN technologies - 100Base-T, FDDI and ATM, for instance.

If all you need is Ethernet and Fast Ethernet, though, you're in luck. So-called 10/100 switches are plummeting in price and indeed are fast-approaching commodity status. For a fixed-configuration 10/100 switch, you can buy a lot of backbone switching capacity for as little as $250 per 100M bit/sec port.

Expect to pay a good deal more for a modular and multislot switch, which lets you swap out some or all ports for different speeds, media, and even different high-speed LAN technologies.

If you're planning to span end-to-end distances of more than about 100 meters with 100Base-T, you'd better plan on at least some 100Base-FX ports. That's the Fast Ethernet equivalent over a pair of multimode optical fiber strands.

A 100Base-FX link will take you a little more than 400 meters in "normal'' Ethernet/Fast Ethernet mode, called half-duplex. For longer distances, up to a couple of kilometers, you'll need to set the link to full-duplex.

And be sure to check the switch's media access control (MAC)-address table capacity. The switch should be able to concurrently maintain as many different MAC addresses as can be expected to communicate through the switch over the course of a few hours on a busy network.

The main reason for this is because all switches treat "address unknown'' packets like broadcasts, flooding them out on all ports. You'll often save bandwidth in the long run if the switch takes a few microseconds more to look through a large address table.

Indeed, broadcast and multicast packets are the Number 1 nemesis of switched networks. There are only two ways to constrain their proliferation - subdivide your switched network with strategically placed routers or deploy virtual LANs.

Be aware, though, that no two vendors' VLANs are exactly the same. And for now at least don't expect vendors' VLANs to interoperate.

The intricacies of VLANs are too numerous to discuss in detail here. But as a potential buyer of switches, be sure of the following:

• Have some idea what your broadcast/multicast packet load is,and subsequently, whether you're likely to need VLANs to control this traffic.
• Thoroughly understand how the particular switch vendor's VLANs work and whether the VLAN structure (port-based or MAC address-based) suits your requirements.
• Exercise the VLAN management application offered by the switch vendor. If it takes more than a couple of minutes to move a station from one VLAN to another, consider the additional administrative burden on you and your staff.

VLANs are one of the aspects of LAN switches that aren't standard today (although progress is being made). But there are other key areas related to switching that the standards don't yet address. These areas include load balancing over multiple links and redundant failover of parallel links - both of which can improve reliability of your switched network.

The bottom line: The more state-of-the-art your switched network becomes, the more likely it will incorporate vendor-proprietary capabilities and features. The trade-off for advanced functionality, then, is interoperability. But isn't that usually the case in networking?