Why is my connection only 100Mbps?

This is most likely a problem with the cabling at some point. Here are the most common problems I have found to cause this in my experience (without certified test results on the installation, which I presume you do not have):

• Improper termination of the cable to the jack/plug.
• Physical problem with the cabling, often the result of the cable being pulled too hard during installation.
• Physical problem with the jacks or plug. This may be a manufacturing defect or the result of improper testing (I have often seen the pins in a jack bent when subject to the "probe" when toned out).
• One of the network cables you are using to connect the computer to the wall could have a problem (this is far more likely if the cable is hand made or older).
• Every once in a while you may come across a network cable that only has two pairs. They were sometimes manufactured this way because it was cheaper to do so.

While 10-BaseT and 100-BaseTX use only two pairs, 1000-BaseT uses all four of the pairs found in your standard Cat5e or higher cabling. If any one of the eight wires has a fault of some sort, this will prevent your connection from negotiating to 1000-BaseT and result in a 100-BaseTX connection.

Gigabit Ethernet is achievable on a cat. 5e network. Without any problems (I have seen it run on cat 5 as well). This behaviour is most probably caused by a breach in some of your pairs. That's why auto-negotiation falls back to 100BaseT