Two switches with the same model number can move air in opposite directions. Install the wrong one and it breathes its own exhaust, or the hot aisle's, and runs hot. The difference sits at the end of the part number: DCS-7050SX3-48YC8-F versus DCS-7050SX3-48YC8-R, or EX4600-40F-AFO versus EX4600-40F-AFI. The definitions below are quoted from Arista's 7050X4 preparation guide and 7050X3 datasheet, and Juniper's QFX5120 cooling system documentation.
What the suffixes mean
| Suffix | Vendor's name | Air enters | Air leaves |
|---|---|---|---|
| Arista -F | Front-to-Rear | Port side | Rear (fans and power supplies) |
| Arista -R | Rear-to-Front | Rear | Port side |
| Juniper AFO | Air Out, front-to-back | Front panel (ports) | Rear panel |
| Juniper AFI | Air In, back-to-front | Rear panel | Front panel (ports) |
Arista describes -F as "port-side air intake" and -R as "port-side air exhaust". Juniper's AFO units take air in "through the vents on the front panel" and exhaust it "through the vents on the rear panel"; AFI units do the reverse. So Arista -F matches Juniper AFO, and Arista -R matches Juniper AFI.
An Arista product number ending in # instead of -F or -R is sold with configurable fans and power supplies, chosen at order time.
How to choose
The rule is the same for every vendor: the side that takes air in must face the cold aisle.
- Ports face the cold aisle (the switch faces the same way as your servers' fronts): buy -F or AFO.
- Ports face the hot aisle (common for top-of-rack switches cabled to the backs of servers): buy -R or AFI.
Check which way the ports will face before you order; it is the one thing that decides it.
Every module must match
Airflow is set by the fan modules and power supplies, and they all have to agree. Juniper is explicit: do not mix fan modules, power supplies, or fans and power supplies with different airflow labels in the same chassis, because it "hampers the performance of the cooling system of the switch and leads to overheating of the chassis". Arista's guide says to verify that each module has the same airflow direction.
This matters most when you buy spares. A replacement fan or power supply has its own airflow direction and has to match the switch it goes into.
How to tell by looking
- Arista: module handles are colour-coded. In the 7050X4 guide, a red handle marks an air-exit module (air leaves through the rear, so port-side intake, -F) and a blue handle an air-inlet module (air enters through the rear, -R).
- Juniper: modules carry AIR OUT (AFO) or AIR IN (AFI) labels. On the QFX5120, AFO is coloured Juniper Gold and AFI Juniper Azure Blue.
Before you buy
- Decide which way the ports face, then pick the suffix from the table above.
- Make sure every fan and power supply in the listing carries the same airflow direction.
- Order spare fans and power supplies with the same suffix as the switch.
Where to look
Townsend lists airflow twins side by side, for example the EX4600-40F-AFO and EX4600-40F-AFI, the QFX5120-48Y AFO and QFX5120-48Y AFI, and the Arista DCS-7050SX3-48YC8-F and DCS-7050SX3-48YC8-R. Every unit is tested before it ships and carries a 3-year warranty. Not sure which direction your rack needs? Send us the requirement and a technician will answer.
