In the unlikely event that someone else that sees this has a Cisco 891 Integrated Services Router - the fan pinout is proprietary if you're looking to silence it.

The regular PC fan pinout is this (for 3-pin):
1. Ground (GND)
2. 5V/12V
3. Tachometer

Cisco's is this:
1. 12V
2. Ground (GND)
3. Tachometer

If you are replacing the stock fan with a Noctua NF-A4x20 FLX, it has a diode to protect against reverse polarity. If you're not, assume it doesn't. Re-pin it and switch 12V and GND first!