Related article: Silencing a Cisco 2951 router.

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I recently purchased a Cisco 2911 to replace my 1921 for use at home, as I needed an extra WIC slot. Now that they’ve been obsoleted by the ISR 4000 series, they’re starting to appear on eBay for relatively palatable sums. For me, the 2911 was a good choice because it has four WIC slots and fits in a 450mm deep rack, whereas the 2901 requires at least a 600mm deep rack, which is far too large for my home office. The 1941 was another possibility, but it’s not enough of an upgrade, and quite frankly, too damn ugly.

Without even having to bother plugging it in and switching it on, I know this thing is going to be too noisy for a home environment. The good news is that the standard array of leaf-blower strength fans are only needed when this product is used in extreme situations, i.e. loaded up with a four WIC cards, a 24-port Gigabit switch service module, with PoE, all ports at full power, and roasting in a street cabinet on a searing hot day in Egypt.

As this does not remotely resemble my use case, I can do away with most of the cooling. First stop – the fan module:

Top is the original, which I am going deaf just looking at, and below is my modified module.

I’ve removed all four of the original fans and fitted a single 70mm 4-wire fan (Delta AFB0712HHB). In order to prevent the system log from filling up with warnings about failed / missing fans, I’ve connected the tach signal from that one fan to the input for the 3 fans.

A quick run of ‘show env’ reveals that this has done the trick. The router being none the wiser to three of the fans being absent.

Just in case it isn’t obvious – the pinout for that connector (Molex 44133-1208) is as follows:

• 1 TACH – 40mm
• 2 TACH – 70mm (rear-most fan)
• 3 GND + Orange presence strap
• 4 TACH – 70mm (middle fan)
• 5 TACH – 70mm (front-most fan)
• 6 GND (middle and front-most fan)
• 7 PWM – 40mm
• 8 PWM – 70mm (rear-most fan)
• 9 +12V
• 10 PWM – 70mm (middle fan)
• 11 PWM – 70mm (front-most fan)
• 12 Orange presence strap

Fortunately I have the Molex Micro-Fit 3.0 contact extraction tool, and crimp tool handy making this easy, but failing that a new connector and contacts could easily be purchased from Mouser.

In my setup, everything runs from a single battery backed regulated DC +12V source. This is no coincidence, as most I.T. equipment internally runs from +12V, meaning that in almost all cases my gear doesn’t require an internal power supply. This router is no exception, needing only a single +12V source (with 5V standby voltage), so I effectively don’t need the power supply here either.

Good news for this conversion, because that’s another source of heat done away with, in fact it means that I don’t need any cooling in the lower half of the router, so that inlet vent can be blanked up – focusing the cooling Mojo of my single 70mm fan solely on the top (mainboard) half of the router.

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But it’s not quite that simple. On my previous router (a 1921) the +12V could be feed straight through to the mainboard with no extra components. On the 2911, we need a bit of extra stuff to satisfy it.

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I whipped up a small emulator PCB which fits in place of the power supply’s original PCB, and has all the extra bits needed to satisfy the routers’ software / hardware – i.e. present its’ self as a PWR-2911-AC, leaving the router none-the-wiser to the fact that it is now powered by an impostor power supply. The downside is that there is nothing but empty wasted space in the lower half of the router.

I’m not going to go into the details of this, but you can download its schematic here. While I was at it, I moved the power switch and inlet to the rear and blanked up the front. A little more convenient, because it means I don’t have to grope around in the back of my rack. For anyone else with the desire and patience to construct an emulator board like mine, a 60W power brick can easily replace the internal power supply.

A quick check shows that IOS is happy with my phony power supply, with the temperature sensor working, serial number and model number still reading as per the original AC supply this replaces.

The power consumption of an idle unloaded 2911 at the 12V stage is 1.8 Amps (about 23W) – show environment reports a lot higher (38W), I am assuming this takes into account inefficiency in the power supply.

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If we are to assume that this is also the unit TDP – It’s practically bupkis given its large size. According to my scientific ‘finger on heatsink’ tests, all of my WIC cards run very cool. The mainboard ASIC also barely gets warm to the touch.

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The only thing I need to keep an eye on is the CPU temperature. The CPU in my unit is a Cavium Octeon (MIPS64), which is fairly energy efficient, but still chucks out the loins share of the heat. It has an internal temperature sensor, which we can read out with the ‘show environment’ command.

At 61 degrees, it is 2 degrees hotter than it was with the stock hurricane grade array of fans, where it sat at 59 degrees. Suffice to say that for my light use case, those fans are indeed completely unnecessary.

For anyone thinking of attempting this…

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Having a single fan is ideal, because there is no risk of irritating ‘beat patterns’ (which often occur when fans rotating at similar speeds are near each other) – but you can only get away with a single fan if also doing away with the power supply, there’s nothing in the service module bay, and the inlet for the lower half of the router is blanked up. As is the case with mine.

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As the PWR-2911-AC does need a little bit of airflow at 30-40 watts, I would suggest replacing with three thinner 70mm fans (like the one I have used) and doing away with / blanking up the 40mm fan, because quiet 40mm fans have so little airflow they’re not really worth it, then strap the tach signal for the 40mm fan to one of the 70mm fans to eliminate software errors.