Active vs Passive Cables

Thursday September 22, 2016

I’ve recently been involved in some design work for upgrading some switching in a data centre, specifically in the Nexus Top-of-Rack range (the 9000 series in this case).

While looking into the best options for deploying the vPC peer-link, I was looking at 40Gb QSFP Twinax cables on Cisco’s site:

Cisco 40GBASE QSFP Modules Data Sheet

In the Ordering Information, you can see that the twinax cables can be either active or passive. I don’t dig into cabling that often, so I had to give myself a quick refresher on what this is, and whether it means anything to me.

A quick explanation is that a passive cable does not draw power from the switches, while active ones do. This is due to signal strength. These type of cables need a signal boost when they get to a certain length. In short, 5m or less are passive cables, over this require the cable to be active.

So does this mean anything to us? In the case I mentioned earlier, no, it doesn’t matter. I need a 0.5m or 1m cable, which comes as passive only. No decisions to be made here.

But what if we were cabling between racks? Depending on the length cable run, we may be moving into ‘active’ territory (which come in 7m and 10m varieties). The price per meter may get a lot higher when active cables are used (especially with breakout cables), so the decision here is whether Twinax cables are the best fit, or whether another option such as fibre is better (that’s fiber in American).

Here’s a couple of additional articles to have a look through if you’re looking for more information:

Difference Between Passive and Active Twinax Cable Assembly

High speed Copper Interconnects Address Critical HPC Hurdles