What are Stackable Switches?

In NETWORKING, the term “stackable switches” refers to a group of physical switches that have been cabled and grouped in one single logical switch. Over the years, stacking features have evolved from a premium (and costly feature) to a core capability of many enterprise-grade switches (and also in several SMB models).

It’s the opposite approach of a modular switch, where you have a single physical chassis with several slots and modules to grow your switch, used typically, at least in the past, in core switches. Both stackable and modular switches can provide a single management and control plane or at least a single configurable logical switch, with some kind of redundancy if you lose stackable switches or a module. Having a single logical switch, with better reliability, makes it easy to translate the logical network topology in physical topology.

What are Stacking Technologies?

In stackable switches, we usually build the stack with cables that connect all the switches in a specific topology. We connect those cables to specific ports of the switches, depending on the type of stacking.

1. Backplane stacking (BPS), where specific stacking modules (usually on the back of the switch) are with specific cables (depending on the vendor).
2. Front-plane stacking (FPS)- VSF, standard Ethernet ports to build the stack, using standard Ethernet cables.

The stacking topology also define the resiliency of the stacked solution, you can have typically different kind of cabling options (depending on the switch vendor and models):

1. Daisy chain or Bus topology do not build switch stacks because it does not provide the desired level of resiliency.
2. Ring or redundant dual ring provide resiliency, but with more than two switches the packet paths can be not optimal
3. Mesh or full mesh provide higher resiliency and also optimal packet paths

To increase the resiliency of stacked switches, there are different solutions based on the concept of a “virtual chassis” with separated management and control planes. Usually, high-end switch models typically implement those solutions.

1. Backplane stacking (BPS)-Vendors utilize specific stacking modules located on the back of the switch, along with specific cables.
2. Front-plane stacking (FPS)-In VSF (Virtual Switching Framework), standard Ethernet ports hep to build the stack . This method involves using standard Ethernet cables.

Advantages of Stackable switches :

1. Management Pane: Logical switch view with a single management interface, which  makes the management and operational tasks very easy. By enabling link aggregation between ports of separate physical switches in the same stack, it enhances bandwidth for downstream links. It simplifies network design by treating “multiple cables” across switches as one logical link using link aggregation solution.
2. Less Expensive: They offer a cost-effective alternative to modular switches, while still delivering comparable scalability and improved flexibility. Resiliency and performance can be different (worse or better) depending on the implementation.
3. Flexibility: You can typically mix various port speeds and media types, as well as different switch models with varying capabilities. For example, you can combine switches with PoE functions along with other models.

Disadvantages of Stackable switches :

1. Performance: For SMB use cases, the stack ports and cable speed are enough to provide high bandwidth and low latency. But when speed increases or the stack expands you may increase the latency and decrease the overall performance.
2. Stability: The stackable switch market is very mature and relatively stable. However, each vendor adds its unique set of features and functionalities. Different vendors utilize different types of connectors, cables and software for their stackable switches. This causes requirements to use the same product line of switches to take advantage of stacking (not necessarily the same model, because, for example, in Aruba 3810 Switch Series you can mix different models in the same stack).
3. Resiliency: Depending on the stacking topology, if you have some faults your overall stack may not be operating correctly anymore. So be sure to choose the best topology and ensure higher resiliency on each stack member. For example, using dual power supplies to ensure hardware redundancy. The single management or control plane may also reduce the overall resiliency, but the problem is similar also on modular switches.
4. Manageability: The single management interface is great, but there are also some drawbacks.Expanding an existing stack could cause an extended service disruption, such as when we reboot all the switches to add a stack member or due to a power failure. Second, removing a switch from a stack could be tricky or require a complex process. Last but not least, upgrading the firmware on all the stack members requires a complete reboot of all the switches.

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