Core and data center applications are the strengths of Arista products. The distribution layer is also a good fit. For the access layer, it would be more of a niche product.
We work in the electric industry. These should not be used in a high-heat environment. We like to keep the temperature of the room in the low 60's so they are not suited for our Substation locations due to extreme heat and cold. However, they perform great in a server room.
Dell ProSupport is outstanding. Dell ProDeploy is likewise outstanding. I prefer them even over traditional Cisco support. I've had repeated cases where Dell immediately addressed or fixed an issue with US-based support, including firmware hotfixes. Not even Cisco can boast that.
I have used the Catalyst 6500 series in the past. From my point of view, the Arista surpasses the Catalyst on just about every front. Originally we were planning on implementing the Nexus 7000/7700 series switches for our core. Though a little more feature rich, it did not provide features we needed that the Arista did. The Nexus also was a confusing and complex platform to work with. Also, the Nexus was a significantly more expensive solution. Although very happy with the Arista switches we may evaluate the Aruba HPE 8400 chassis-based switches along with Arista switches in the future.
I selected Dell mainly due to previous experience using them in a business environment. I've never had a unit fail on me and when my budget for equipment like network switches has been less than a training class on how to use those switches, it's incredible to be able to find quality equipment at the price point Dell offers and even more so when their enterprise support has been as excellent as I have been accustomed to dealing with. In all honesty, even if I had a larger budget, I would have bought these same switches since they were exactly what I needed to suit my needs. We looked into HPE/Aruba network switches, and while they would have performed everything we needed and more, they were almost four times the cost of these Dell network switches. That's a hard sell when it comes to planning a tight budget.
