- Distribution : should you be picking a Datacenter with narrow racks, cabling may very quickly become messy. Most people push for top of rack / end of row layout but on some installations this can get very messy and I usually go for middle of rack distribution. Indeed, this shortens all the cables but also allow to only have half of the cables above or below your distribution switch
- In addition to the middle of rack distribution, an important point to assess is the server cabling of your Datacenter. Distribution switches are, as their names indicate, for distribution. As such, they connect all the servers and equipments that have all their ports on the cold aisle side. In order to avoid having massive cable trays and complex management of cabling, I really suggest you study possiblity to have an inverted switch (ports, instead of being oriented towards the cold aisle, would be oriented towards the hot aisle. When turning your switch around to perform this, you have to be sure your airflow is also inverted, so that the switch intake is on the opposite side of the ports. An example of this is Cisco Fabric Extenders (http://www.cisco.com/c/en/us/products/collateral/switches/nexus-2000-series-fabric-extenders/product_bulletin_c25-680197.html) that have this inverted airflow option.
- Cabling plan and ordering : this is usually a critical task on a datacenter build. Make sure a unique company is in charge of the cabling in order to have something consistent but also to have a unique vendor to go back to if you see any issues. Cabling should be preinstalled before the equipments are racked. This is usually done via a cabling matrix. Make sure you ask well ahead if there is a cabling matrix standard to use or if you can provide your own. Should you provide your cabling matrix, you will have to know who is in charge of calculating cable lenghts (should you be in charge, you will need all the details on the rack heights, the distance from the rack top to the cable trays...). The cabling matrix should at least comprize :
- Source equipment name, rack, rack position, port / interface
- Target equipment name, rack, rack position, port / interface
- Cable type (fiber, copper, stack...) and colour
- Cable function (Management OOB network, LAN, SAN...etc...)
- Cable label
- Some switches such as Nexus 5k seem to have a physical depth issue. Indeed, the ears (little "L" shaped metal plates you fix on the side of the switches to rack them) are pushed too far back by default. When checking with Cisco, they do not provide different models of these ears. The only solution is usually to remove 2 screws to shift the ear forwards ans give enough space for fibers to reach your switches in the racks. Please note this is only possible if you have rails to hold your switch at the back, as without rails, you will face serious issues. On the picture below, left side, you see the switch with ears put in initial position, and on the right once 2 screws are removed and the ears repositionned.
