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How to choose a data center switch

Choosing the right data center switch is critical if you want to keep your network engineer happy. There are two sentences he or she never wants to hear:

  1. “The data center switch is the bottleneck” and
  2. “You have to work this weekend to change out the switch.”

Oddly, choosing the right data center switch for a network engineer is both a business and a lifestyle decision. There are several types of data center switches available, which means choosing the right switch is not easy:

  • ToR (Top-of-Rack)
  • Aggregation
  • Other switch likely at a remote location

Here are five important factors to keep in mind as you pick the switch.

1. Reliability 

The most important factor to consider when buying a data center switch is reliability.  More money does not necessarily mean reliable. Nor does buying new instead of seller refurbished.  For new, there are issues with unforeseen bugs from the OEMs (original equipment manufacturers).  Cisco has had clock issues, fan issues and firmware bugs that have made many of their new switches and devices unreliable from the start.  Known good units can be purchased easily from used resellers. Finding out about known bugs or system errors is very important before choosing. 

Tested seller refurbished switches tend to have just as long a lifetime as new switches and do not have any of the “wear-in” issues that newer switches may have.

ToR switches tend to be the best choice for managing pure data movement within a rack of servers.  These switches tend to single purpose designed for moving data a very high speed.  Arista, Juniper, Cisco, Mellanox, Gigamon, and HPE are known for ToR switching.

2. Physical size of data center switch

While the physical size of the data switch would not seem to matter, it will affect the resources you will need to install it, to power it, to change it or to support it.  The larger the switch, the more people it will require to put into the rack.  The larger the size, the more pieces of hardware it will require ro rack and to and the more one will have to plan cooling to ensure that failure due to a hotspot does not occur.

Choosing a smaller data center switch may mean that you will have to design data flow along the top of the racks to keep the data from bottlenecking between cabinets.  One might consider using Infiniband switches for between cabinet speeds if the need arises.

Deploying a large number of small data center switches is a more power efficient to an arrangement of fewer but larger units when the traffic demand is moderate and the servers exchanging traffic are in close proximity. 

3. Switching capacity and latency

Two critical measures of a data center switch are switching capacity and latency.

The switching capacity or backplane bandwidth of a switch is the maximum amount of data that can be transmitted between a switch interface processor or the interface card and a data bus. The switching capacity indicates the total data exchange capability of the switch.  This is usually defined in gigabits-per-second.  The higher the switching capacity of a switch, the stronger the ability to process data, but also the higher the cost of the data center switch.

Switch latency is the amount of time that an Ethernet packet spends traversing an Ethernet switch. This is measured from port-to-port on an Ethernet switch. It may be reported in a variety of ways, depending on the switching paradigm that the switch employs: Cut-through or Store-and-Forward.  Generally the OEM will define the switch latency in one or the other method.  Make sure where comparing latency figures that you are comparing by the same method.

4. Cooling and air flow

You have selected your data center switch, have figured out the rack design, you have decided on cabling and are ready for install.  But if you go through all that work and you install the data center switch in a location on the rack that won’t ensure that the switch will stay cool, you will have lost the battle.

The other part about cooling is air flow or air direction through the switch. Data center switches come with either a Front-to-back or back-to-front air flow.  Air flow is really determined by where you are going to put your network cables.  If the servers have the network connections on the back of the servers, you really want to have back-to front airflow as you should install the switch “backwards” so that the air comes in from the power supplies on the cool side of the aisle.

If the network connections are in the front of the servers then choose front-to-back cooling and very carefully arrange the cables so that the airflow into the switch is not blocked.

Cables that block air flow – even a little bit – can cause the switch to fail early.  Within a data center, air flow determines the life of your switch.  A small disruption in air for a few days does not matter.  A data center switch may not move for years and that small disruption may mean one chip on the board does not get enough air flow and it may fail faster.

This article expands more on that idea: why data center airflow management is also a critical factor for your data center.

5. Vendor support for your data center switch

Support for your data center switch the OEMs is equally important. Be sure to choose a reputable vendor with a good reputation. Evaluate their services in terms of hardware replacement, software upgrades, configuration, and troubleshooting assistance.  An easy test is go to their website and check the options for downloading newer versions of firmware.  

Arista and HPE offer tremendous customer support.  Cisco is often distant in the support area as their support is only available if one has their switches under a Smartnet agreement.

Determine the vendor’s credibility by checking out testimonials from other customers and research their policies to see how responsive they are to customer inquiries. A help hotline is of little use if no one is at the other end. 

Look at various web sites to see what third party options for support you may find if the OEM is not available.  Sometimes, third party support is both better and faster than OEM support.

Remember the rules when buying a data center switch

  • Reliability means more than buying new or paying high prices.
  • Pick the size of switch that is both easy to install support and maintain
  • Choose the correct switching capacity and latency for your needs
  • Cooling and air flow are very important
  • Plan your support before purchasing your switch

Source

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

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