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Stackable switches: to stack or not to stack?

In the early 2000s, stackable switches did not exist. The only way to offer a large number of users a simple network topology was to use modular switches in one or several clunky fridge-sized switch chassis.  These were loaded with network modules and system management cards.  They required a cabinet, cooling fans and redundant power sources.  As a network manager, the fear of blowing electrical circuits and cutting power to one’s business when powering up these switches was real.

In 2005, when Cisco released the Catalyst 3750 series, it offered an alternative to the clunky modular switches.  These “pizza box” style switches came with a stacking port that allowed a limited number of the normal sized switches to be stacked together to create a much larger virtual switch.  Essentially, the stacking port was a direct tie to the backplane of the switch and by connecting to the backplane, the switches would act as if they were one unit. Shortly after releasing the 3750, Cisco released the and the Catalyst 2960-S, which provided a lower cost option for network managers seeking stacking capabilities without the layer 3 and other higher end features of the Catalyst 3750 series.

These switches drastically reduced the amount of space, power and management that a modular switch required. The technology and idea has advanced through the years.  Now, Juniper, Dell, Extreme, HPE, Aruba and Cisco offer stackable switches.

What are stackable switches?

Simply put: a stackable switch is a network switch that has a backplane that can be directly connected to another switch via a stacking or dedicated network cable.

In networking, the word “stack” or “stackable” describes an association of physical switches that have been connected and ordered in one single logical switch. Over the years, stacking characteristics have evolved from a premium or expensive feature to a standard capability of virtually all enterprise-grade switches and many SMB (small-and-medium sized) models.

Modular switches cannot be externally stacked or connected through the backplane.  One could direct them via a high speed fiber connection uplink but this would not achieve a connection via speeds comparable to a backplane connection.  One could argue that a modular switch is an internal stack where every module connects to the backplane.  The backplane — or the copper wiring or conduit connecting the modules to each other — is a common feature shared with a switch stack.  The difference, of course, is how that connection is made.  The modular switch does it through a large physical backplane whereas the stacking switch accomplished through stacking cables that interconnect smaller backplanes.

Both modular and stack switches offer a single point management interface that treats all part of the module or stack as a single virtual device.  Possessing a single logical switch, with good reliability, makes it easy to manage one’s company network. In a world where network speed is life and the ability to see and correct a bottleneck quickly, a network stack is a great solution.

How does one create a switch stack?

Grab same make and model family switches with relative close versions of the same firmware.  Grab the correct type of stacking cables.  “Round robin” the connections.  If one has the correct units, correct firmware, correct type and number of cables, then the stack will work.

  1. Same model family of switches.  Cisco requires that the model the same model family is used in a stack.  For example, Cisco 3650s cannot be stacked with 3850s.Juniper offers a more vibrant option and allows one to stack EX2200, EX2300, EX3300, EX3400, EX4200, EX4500, EX4550, EX4300, EX4600, or EX4650 switches operating as one logical device and managed as a single Virtual Chassis.  Dell N4032, N4032F, N4064, and N4064F series switches can be mixed in any combination within a stack.  The underlying challenge for the network manager is to ensure that all the switches have nearly the same firmware.
  2. Matching firmware:  Cisco 3750x can only be stacked with 3750x switches having the same version of the firmware. One should strive to make sure that all switches in the stack have the same firmware and the same version.  Lan-base switches should only stack with other Lan-base firmware switches.  Juniper switches work in the same way.  One must have the same (or very close) version JunOS on each switch or the stack will not work.
  3. Correct cabling and connections.   Before doing a quick look at cabling and cabling options, most important to understand the three different cabling approaches to a switch stacking solution. These are:
    • Backplane stacking (BPS): A kind of stacking where special stacking modules (usually on the back of the switch) are used with specific cables, depending on the manufacturer.
    • Front-plane stacking (FPS): Uses standard ethernet ports to create the stack with standard ethernet cables.
    • Power stacking: Power supplies can be virtually connected within a stack of switches to create a virtual power supply for the stack.
    • Cisco Example:  Cisco is known for using predominantly back-plane stack family of stackable switches:  Cisco Catalyst 9200, 3750G, 3750X, 3650, 3850, C960S and 2960X require different types of stacking cables and may require stacking modules for the stack to work.  The Catalyst 9200 uses Type 3 such as the STACK-T4-50CM.  The Cisco Catalyst 3650’s, 2960S’s, and 2960x’s do not have any built in stacking options.  One must buy the modules separately.Those parts are C3650-STACK-KIT, C2960S-F-STACK,and C2960X-STACK.  For cabling, the 3750G, 3750X uses CAB-STACK-50CM and longer options.  The 2960S/X uses FLEXSTACK or CAB-STK-E-0.5M and longer options.  The 3850s use STACKWISE-480 or STACK-T1-50CM and other longer options.
    • Dell, Juniper and HPE switches use either front-plane stacking or backplane stacking.  As the varieties depend upon the model and family, best to read the setup manual or the datasheet to understand the stacking options for each module
  4. “Round-robin” the connections. For a switch stack to work properly, for all manufacturers that we have observed, one must “round-robin” the switch stack cabling.  In a three switch stack, for example, this means that switch 1 must be connected to switch 2, switch 2 to switch 3 and switch 3 to switch 1.  This “round-robin” provides the redundancy necessary to keep the stack up and running should a switch, a power supply or a cable fails.
  5. Power Stacking.  Power stacking is an option to cross connect power supplies in switch stack so that if one power supply fails, the other power supplies on other switches can power the switch with a bad power supply.  The Cisco 3750X amd 3850 use the PowerStack cables such as the CAB-SPWR-30CM and CAB-SPWR-150CM. If each switch in your stack has dual power supplies, then you would not likely need to opt for power stacking.

The stacking topology also specify the resiliency of the stacked explanation, you can have various kind of cabling alternatives depending on the switch manufacturer and model.

When should you choose to stack switches?

Stacking is more or less a corporate infrastructure option.  You should choose to use it when you want to create a resilient network connection that has limited single-point-of-failure.

Advantages to stacking:

  1. Virtual Chassis management: A switch stack appears as one single “virtual” stack.  For a network manager, this makes stack easy to manage and easy to configure.  One can also better see any bottlenecks or slow downs within the stack.
  2. Different Port Speeds in the same stack:  As long as the switch is from within the model family with most vendors, one can add switches with different port speeds and configurations.  As mentioned above, Juniper’s EX Virtual Chassis option does not have this limited option.
  3. Easier to Scale or Replace:  For small switch stacks, if one wants to upgrade the switching capacity and backplane speed of a modular switch, one must change out the chassis. As a network manager, one would have to plan weeks if not months in advance to do that work.  With a switch stack, one can upgrade a switch at a time without planning a major outage.
  4. Lower costs than modular options:  For small stacks, much easier and less costly to assemble a three switch stack than it would be to set up a modular switch with all the parts necessary with similar redundancy.
  5. Deployment flexibility: switches can be used either in a stack or as independent switches in a separate deployment

Some disadvantages to stacking:

  1. Large Stack Management: The larger the stack the more difficult one will find changing out one switch within the stack due to the more complex nature of a large stack.  In some cases, the entire stack may have to be shut off to replace one switch.
  2. Power management:  Large dual power supply PoE+ switch stacks can consume far more power than a modular switch offering the same number of ports and the same switching capacity. Although 3750X and 3850 switches offer power stacking options, sometimes, this option may not work given space and cabinet constraints.
  3. Switching Capacity:  Even with the high speed of the 3850 and 3650 switches, the switching capacity of a switch stack may not approach that of a modular switch.  Stacking switches may mean some loss of switching capacity.  One must study the pros and cons of the switches one is choosing rather than blindly opting for a stacking solution.

A simple rule?

Stacking for the most part is a corporate infrastructure decision as one may likely be managing PoE devices and people working within an office environment.  If one is designing for a large number of people, one must consider the size of the stack.  If you are designing at the large end of a stack size, consider using a modular switch.  If one is designing at the small end of the stack size, choose to use a stacking solution

References

Mike Syiek
Mike Syiek
Mike Syiek is President and Founder of NetworkTigers.

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