Cisco 3945

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I updated my ancient infrastructure to less ancient Cisco 3945 ISR. The Cisco 3945 is rated at 1500 mbit/s of IMIX NAT performance, and works well for my 700/700 PON circuit at the house. On the plus side, it will take the "SM-X-ES3-24-P: EtherSwitch SM L3 + PoEPlus + MACSec + 24 10/100/1000" modules, providing POE+ and GigE support[1]. These modules link into the Multi-gigabit Fabric (MGF) allowing them to interact with the router at the VLAN level and other ports. It's a bit nicer than having to do sub-interfaces in the router IOS. The switch also runs the 3560E code and support VLAN routing, and presents it self as a separate switch to the network.

One issue I ran into, is even shut away in a network closet, the fans scream in the router. The standard fan is a Delta QFR0812UHE, and is rated for 9000 RPM and 105 CFM! Considering the 2 sqft of space in the router, and the 5 fans, this means they are able to move 262 times the volume of the chassis through it every minute.

As these are a standard 80mm PWM fan, I found they could be converted to use a smaller/slower fan, yet still keep the chassis cool (in my environment) and IOS happy. I found a 3000 RPM fan the ARCTIC P8 on Amazon in a convenient 5 pack for under 30 dollars. While not the same depth as the OEM fans, they fit and seem to work well.

these might be better, if you're intending to load more stuff in it.

IOS has them running at 62% PWM on Level 3 (of 5) at 2100 RPM in my use. This leaves enough headroom if they have to go to full speed, and still provides a 50x airflow volume to the chassis. It's not going to meet NEBS anymore, but I don't live in a CO (unlike some people).


As of April 2022, I've gone with these fans from Amazon, and the exhaust temp is way down.

I've also found that removing the pop rivets holding the bracket that separates the two lower module bays allows inserting another double wide card and it works.

Notes on the process

The Fan module is a huge pain in the ass and sharp. The fans are hardwired to the connector, but do use the standard Red, Black, Yellow, Blue PWM colors. I elected to install connectors to accept standard fans, to make changing them easier. Now the right way to do this would have been to buy the connectors and pins, then crimp them on, but I didn't want to wait to order them, or even research what I needed. I had some PWM fan extenders on hand for another project and elected to splice them in on the existing fans unit. This allowed me to use the other end on the old fans for another project :D


So I tried to do this fast with the router running, I mean it can't get that hot, right?

It went into thermal shutdown on the PSU's in 25 minutes. Funny thing, if it does this, just putting the fan unit back on won't fix it, as they are shut down, and can't power the fans. You either have to wait for it to cool off, or blow some cool air into it.

I found the air intake temp went up about 10 degrees C and the exhaust air was much hotter, closer to 50C. The Delta T went from 4-5 degrees C (Input - Output), to 15-17 degrees C. This is due to it moving much slower in the chassis, but it's still cooling and the PWM is running at 62%. If it overheats, it will shut down, and if it's that hot in the house, I'm not here anyways!

#sh environment table
 Internal Power Supply 1 Type: AC-POE
 Internal Power Supply 1 12V Output Status: Normal
 Internal Power Supply 1 POE -48V Voltage Status: Normal

 Internal Power Supply 2 Type: AC-POE
 Internal Power Supply 2 12V Output Status: Normal
 Internal Power Supply 2 POE -48V Voltage Status: Normal
 System Power Supply POE Mode: Redundancy

 Fan 1 OK, fan level 3, Medium speed setting, 2152 RPM, 62% PWM
 Fan 2 OK, fan level 3, Medium speed setting, 2141 RPM, 62% PWM
 Fan 3 OK, fan level 3, Medium speed setting, 2085 RPM, 62% PWM
 Fan 4 OK, fan level 3, Medium speed setting, 2104 RPM, 62% PWM
 Fan 5 OK, fan level 3, Medium speed setting, 2166 RPM, 62% PWM

 Intake Left temperature: 37 Celsius, Normal
 Intake Right temperature: 34 Celsius, Normal
 Exhaust Right temperature: 50 Celsius, Normal
 Exhaust Left temperature: 54 Celsius, Normal
 CPU temperature: 37 Celsius, Normal
 Backplane temperature: 36 Celsius
 Power Supply Unit 1 temperature: 42 Celsius, Normal 
 Power Supply Unit 2 temperature: 39 Celsius, Normal

So that's all


  1. to use the POE you need the power supplies with -48v in them