Buying a Stackable

FRAMINGHAM (03/13/2000) - Stackable switches are popular - particularly for e-commerce applications - because stackables offer rapid deployment, good value, scalability and ease of administration.

However, evaluating a stackable switch purely on price per user port doesn't tell the whole story. In fact, judging a product on that factor alone might be misleading if a vendor calculates price per port or makes other claims based on a configuration that doesn't square with real-world conditionsA more thorough way to evaluate stackable switches is to add scalability and manageability to your checklist.

When it comes to scalability, many vendors simply list a large number of user ports to substantiate their claims for high scalability. For example, Lucent Technologies Inc. claims the highest number of ports, but with a configuration that has no uplinks. The important issue is determining where blocking, or saturation of the switch, occurs. There are three key points to consider:

* Internal scalability: To what extent do the interconnections between stack units limit scalability?

* Bandwidth growth: How many ports can be upgraded from 10M-bps Ethernet to 100M-bps before the switch becomes overloaded?

* External scalability: To what extent does the switch uplink limit traffic to the rest of the network?

As an example, let's take a switch with 24 user ports - we're using the term "user ports" deliberately because in a real-world configuration a certain number of ports are already used for server or internal and external connections.

If the 24 ports were all carrying 10M-bps traffic and if the stackable unit were connected with a Gigabit Ethernet uplink, then upgrading eight ports to 100M-bps Ethernet would saturate the uplink.

The math works like this: The eight ports at 100M-bps equal 800M-bps of traffic, and the remaining 16 ports at 10M-bps equal 160M-bps. That total of 960M-bps means no additional Fast Ethernet upgrades can be accomplished without risking blocking on the 1G-bps uplink.

Following the same formula, if the uplink were 2G-bps, the switch could handle 19 Fast Ethernet ports before blocking occurred.

This same exercise applies to uplinks from the stack to the rest of the network.

We sampled a half-dozen stackables: In the Layer 2 category, we looked at the 3Com SuperStackII 3300, the Cisco Catalyst 3524XL and the Nortel Networks BayStack 450-24T. In the Layer 3 category, we looked at the Extreme Virtual Chassis, the Lucent Cajun P330R and the Network Peripherals Keystone 24mg.

For a standard configuration, we chose 96 user ports, two Gigabit Ethernet uplinks for connecting the stack to the network, and the vendor's own stack interconnect.

We found that all the stackables had bandwidth to spare for their full complement of Ethernet ports. But when it came to scaling up to Fast Ethernet, only the stackables from Network Peripherals, Lucent and Nortel could handle all ports at Fast Ethernet speeds without causing bottlenecks between stack units.

Furthermore, if you're considering a Layer 2 stackable, be aware that you''ll need to add Layer 3 functionality - either with a connection to an external router or Layer 3 switch, or as a blade for the Layer 2 switch. However, at this point many Layer 3 stackables also do not yet support Differentiated Services, type of service or Multi-protocol Label Switching. The stackables also would need an external router or Layer 3 switch to achieve the management features that you're looking for.


Numerous studies have shown that costs related to operations and management consume larger amounts of money over a product's lifetime than the initial purchase price. Thus, manageability becomes another key element in evaluating overall value.

The inherent advantage of a stackable switch is it's easier to manage a single logical entity rather than multiple devices that must be configured and monitored separately.

But there are other factors to look at, including quality of service (QoS) for handling prioritized traffic, the ability to enforce policies and the ability to manage virtual LAN traffic, as well as ease of deployment and operations.

QoS features focus on reserving needed bandwidth and forwarding traffic to support different service level needs. All products we looked at support the IEEE 802.1p and 802.1Q standards for describing priority and VLAN membership.

But the results are mixed when it comes to support for Resource Reservation Protocol, the common mechanism for reserving specified bandwidth for a specific flow. Reservations are necessary to ensure that adequate bandwidth is available when a connection is created.

Another area to examine is support for policies, which are rules that control switch behavior. Network administrators use policies to allocate bandwidth, assign priority to an application flow and control network access. The focus is on bandwidth management policies needed for meeting service-level agreements.

One significant factor to look at is the means for distributing policies to the switches. Policies must be distributed to groups of switches because configuring one switch at a time is labor-intensive and introduces the potential for making entry errors. Large numbers of switches can be changed very quickly when new policies are required, so check to see whether the stackables support Directory Enabled Networks and Lightweight Directory Access Protocol. Also ask the vendor whether it plans to incorporate Common Open Policy Services, a richer protocol that's expected to follow.


Customers use VLANs to manage traffic across a set of switches, or stack units.

All vendors support VLANs defined with 802.1Q tags. Most support other VLANs based upon switch ports, media access control address, Layer 3 protocols or policies.

When it comes to general operations, be sure to look at features that simplify configuration through automatic discovery or through a directory-centered approach. For example, Lucent offers a feature that automatically builds routing information from a discovered router. Lucent also is the first company to offer switch monitoring to supervise multiple stacks.Price per port is one factor to consider, but it's also important to evaluate scalability and manageability before you buy a stackable switch.

John McConnell is president of McConnell Associates in Boulder, Colo. He can be reached at

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