Silencing a Cisco 2911 router for home use

Related article: Silencing a Cisco 2951 router.

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.

 Fan 1 OK, Low speed setting
 Fan 2 OK, Low speed setting
 Fan 3 OK, Low speed setting
 Fan 4 OK, Low speed setting

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.

Eliminating the power supply as a heat source

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.

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.

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.

PWR-2911-AC with all of its original internals removed, replaced with a small pass-through / emulator board (blue) to allow 12V operation

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.

Router#show inv
NAME: "C2911 AC Power Supply", DESCR: "C2911 AC Power Supply"
PID: PWR-2911-AC , VID: V05 , SN: DCA1647R2GF

Router#show env

 Power Supply Unit temperature: 28 Celsius, Normal

From the rear. Showing 12V input feed.

How it runs

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.

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.

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.

 Intake Left(Bezel) temperature: 31 Celsius, Normal
 Intake Left temperature: 23 Celsius, Normal
 Exhaust Right(Bezel) temperature: 34 Celsius, Normal
 Exhaust Right temperature: 27 Celsius, Normal
 CPU temperature: 61 Celsius, Normal
 Power Supply Unit temperature: 28 Celsius, Normal

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…

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.

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 you are not going to find a quiet one, then strap the tach signal for the 40mm fan to one of the 70mm fans to eliminate software errors.

10 thoughts on “Silencing a Cisco 2911 router for home use”

    1. I wouldn’t have written this article if it didn’t!

      It now makes about as much noise as the fan in my laptop.

      1. Would you consider making a couple of these for sale? I’ve got 2 cisco 2911’s that I would like to silence. They are loaded up however– what would you recommend?

        Here is my show inv and show env:

        NAME: “CISCO2911/K9”, DESCR: “CISCO2911/K9 chassis, Hw Serial#: , Hw Revision: 1.0”
        PID: CISCO2911/K9 , VID: V06 , SN:

        NAME: “ADSL/VDSL over POTS supporting ADSL1, ADSL2, ADSL2+ and VDSL2 on Slot 0 SubSlot 0”, DESCR: “ADSL/VDSL over POTS supporting ADSL1, ADSL2, ADSL2+ and VDSL2”
        PID: EHWIC-VA-DSL-A , VID: V01 , SN:

        NAME: “ADSL over POTS on Slot 0 SubSlot 1”, DESCR: “ADSL over POTS”
        PID: HWIC-1ADSL , VID: V01 , SN:

        NAME: “WAN Interface Card – One Port Analog Modem(Version 2) on Slot 0 SubSlot 2”, DESCR: “WAN Interface Card – One Port Analog Modem(Version 2)”
        PID: WIC-1AM-V2 , VID: V03 , SN:

        NAME: “SM-ES3G-16-P: EtherSwitch SM L3 + PoE + 16 10/100/1000 on Slot 1”, DESCR: “SM-ES3G-16-P: EtherSwitch SM L3 + PoE + 16 10/100/1000”
        PID: SM-ES3G-16-P , VID: V02 , SN:

        NAME: “Internal Services Module with Services Ready Engine on Slot 0”, DESCR: “Internal Services Module with Services Ready Engine”
        PID: ISM-SRE-300-K9 , VID: V02 , SN:

        NAME: “C2911 AC-POE Power Supply”, DESCR: “C2911 AC-POE Power Supply”
        PID: PWR-2911-POE , VID: V02 , SN: AZS16****PK

        Internal Power Supply Type: AC-POE
        Internal Power Supply 12V Output Status: Normal
        Internal Power Supply POE -48V Voltage Status: Normal

        External Redundant Power Supply is absent or powered off

        Fan 1 OK, Low speed setting
        Fan 2 OK, Low speed setting
        Fan 3 OK, Low speed setting
        Fan 4 OK, Low speed setting

        Intake Left(Bezel) temperature: 24 Celsius, Normal
        Intake Left temperature: 27 Celsius, Normal
        Exhaust Right(Bezel) temperature: 42 Celsius, Normal
        Exhaust Right temperature: 31 Celsius, Normal
        CPU temperature: 56 Celsius, Normal
        Power Supply Unit temperature: 42 Celsius, Normal

        Battery OK (checked at power up)

        Motherboard Components Power consumption = 69.4 WPower consumption =
        ISM 0.0 WPower consumption =
        NM/SM slot 1 27.6 W
        PoE usage 30.8 W
        Total System Power consumption is: 127.8 W

        Environmental information last updated 00:00:16 ago

        1. You aren’t going to achieve the same kind of silencing I did. You definitely could replace the three 70mm fans with slimmer slower ones like on mine, and remove the 40mm, connecting its tach signal to one of the 70mm fans.

          That will make a big difference to the noise output, and shouldn’t be too difficult to do.

          I don’t sell products or services. Everything I put up here is more of a how-to guide.

  1. Very helpful Matt! Thank You!!! I my fan #4 shit the bed today. Worse it was gradually spooling up and down all day, then it died and told the other three 70mm fans to spool up to full RPM. I found your page right in time. I spliced the wiring from Tach and PWM from the 40mm into the 3rd fan (70mm) wiring and viola!

    Thanks a million!

  2. Thanks for taking the trouble to post this. My 40mm Fan 4 was stopped and all temps were still normal so I just wired it’s tach pin to a good fan using your pinout diagram and it’s quiet again. Thanks.

  3. Great post. Am wanting to do the power supply conversion to 12v DC. i Dont have the skills to make a PCB. What 60W PS to use and any other components shown for the PSU image, you posted above? Good job.

    1. Honestly I think you can get away with applying 12V onto the power pins the power supply normally mates with. IOS won’t like it – you’ll get the fault LED blinking, and endless error messages about communication failure with the power supply which you won’t be able to silence. But it will work.

      If you want something a little better – the most important components (which have to be lifted from a donor power supply) are U1, U5 and U7. /OUTPUT_OK and /INPUT_OK can be tied low if you’re looking for shortcuts.

      The mating connector 51730-146 is unobtainable – you’ll have to lift that from a donor power supply – extremely difficult. How hard it is depends on the brand – for example, on Delta branded supplies it’s attached with large amounts of lead free solder with a very high melting point, with through holes on very tight clearances. It’s more or less impossible to remove without destroying it.

      Mine came from a “Quality Components & Systems” branded supply which was a lot easier to deal with – far less solder, and larger through holes.

  4. Did you log pre-modification RPM and post-modification RPM of the case fans? Do you know if IOS will complain if the fan RPM is substantially lower? I’m looking to mod the 3900-FANASSY fan tray in a 3925, same purpose, quieting the turbine in the corner.

    1. That’s a very good question actually – certainly something I was also concerned about.

      The RPM of my new fans is perhaps half the originals, but actually IOS doesn’t care much about this. So long as it’s above zero you won’t see any error conditions.

Leave a Reply

Your email address will not be published. Required fields are marked *