Strange problems with switches

Network switches are supposed to be simple devices that work at TCP/IP layer 1, the hardware layer. As far as the network is concerned, and the nodes on the network, switches should be invisible. And mostly they are.

But — just sometimes — they can do strange things.

The problem

Last week I was working on a little project I’d been trying to get to for several months. I have a small, wall-mount 19″ rack in my garage. Four network cables from my computer lab, which is above the garage, run down into the rack, and several more run out to various rooms around the house. I have a 24-port patch panel to make the appropriate cross-connections and a small 5-port switch in the rack.

The exact details aren’t relevant to this, but I had a very old UPS powering the switch. The UPS was essentially a brick and did nothing more than pass power through to the PDU. Small power glitches always resulted in the switch needing to restart and a new battery didn’t resolve the problem.

The solution

My intention was to replace the old UPS with a new one that used USB instead of a serial cable for management, and to install a 20-year old Dell computer on a new shelf on top of the rack to manage the UPS. I also need to get some of the 13 computers I have in my lab out of there to reduce the heat load. Another consideration is that I plan to have more cables run to other rooms in the house and I needed a new switch with more ports to handle the additional lines.

I bought and installed a new UPS and a 24-port TP-Link switch. I connected the network cable from the TP-Link switch in my lab to the new switch in the rack, via the patch panel. It worked just fine — NOT!

Well it did work, but it only connected between the two switches at 100Mb although my whole network is full 1Gb, including the switches. I could see the speed just by looking at the lights on the routers.

After a bit of detective work, I recalled that the 5-port switch always connected at 1Gb. So I recabled a bit so that the 5-port switch was between the two main switches and both 24-port switches now connect to the intermediate switch at 1Gb.

Why??

I have no idea why this problem occurred. It really seems like that should work just fine. But thinking back even further, I also noticed that in one temporary configuration, I had two 5-port TP-Link switched connected together and they had the same problem and would only negotiate 100Mb between them.

Just in case you run into a similar problem, you’ll have something to try. Let me know if this works for you.