I have what I think is a real head scratcher. I have looked for answers and found none so far. We have had Triple Play for about 7 years and recently upgraded to gig speed. I ran an xfinity speed test as soon as I installed the modem (cable/modem/router combo) and got a 935 mbps download speed. I realized the next day that the phone wasn't working so I called xfinity and had the phone re-activated. A day later when I ran a speed test I got 123 mbps download speed. After several calls with xfinity a service technician came out and checked all of the interior/exterior cables and said all was well so he changed the modem. Once again I got 935 mbps download speed. Right after he left I tried the phone and it wasn't working. I called xfinity, got the phone working, ran a speed test and speed was 85 mbps. Xfinity sent out another tech who attached a cable to the ethernet cable and showed me the ethernet is pulling a gig. As an additional test I hooked up a laptop and got over 900 mbps. He did not know why my desktop computer is slowing the speed dramatically when voice is activated and I have scoured the internet and could find nothing. If anyone has figured this mystery out or even has some things I could try, please let me know. Thanks to all.
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Is your device a modem+router device and is your PC plugged into it? Did your device reboot after enabling the phone? I suspect the answer is yes to both.
Part of bringing a wired connection up is negotiating line speed. The protocol is to start with the fastest possible speed, here 1Gb, and if link can not be established at 1Gb, down-clock to a slower speed, in this case 100Mb. If link is established here it remains @ 100Mb until the connection is restarted by unplugging the cable or rebooting one of the devices.
Another option would be that the ports have been forced to support only 100Mb. If that is the case there's nothing you can do as it would be a result of a config setting.