Okay everyone I really appreciate the replies. I have an update, but first I will respond to some of the points made above. @jlavaseur - I had my router in my config initially, but when I started troubleshooting issues, I completely removed it and just used the XB6 for DHCP etc. This was due to the fact that whenever I called comcast, the extra router would just confuse them. They would start along the line of thinking "IT MUST BE THE ROUTER". So I made it simple for them (and me) by just removing it from the equation. The second part of you question is "why use an extra router". Easy - I work IT and Comcast Business uses a lot of the same modem hardware as residential clients. Business users are very clearly told that the Comcast hardware is not meant to be an enterprise firewall - IE: Don't come crying to us if there is a security issue (aka you got hacked), because you are expected to BYO firewall. Hence, I have a more robust system protecting my network/devices. @EG Yes, for a while, I had it in bridge mode. I also tried in in DHCP, pulling an IP from Comcast. Made no difference. Same slow speeds. NOW FOR THE UPDATE: I got fed up with dealing with Comcast tech support, which only has 4 tools in their belt 1) Reset modem 2) reprovision modem 3) check signal strength 4) roll tech to site Due to the strange issues I saw when plugging/unplugging devices, I went out and bought my own modem; mainly out of curiousity. I got an Arris Surfboard SB8200. After getting it fully provisioned, SURPRISE: 920Mbps/57Mbps. And just to be sure, I had the Comcast phone tech setup the XB6 again which tested at 50-400Mbps. Then, I went back to the SB8200...900+!!! Now, my only issue is that COMCAST only give the $15 XFi advantage unlimited data to their own modems. If I want unlimited data with my "customer owned" modem, I have to pay $50/mo extra. Sorry, that's just nonsensical. If it were up to me, I would have the XB6 - it's not my fault I have to use a non-Comcast modem. I shouldn't have to pay $50 lol. Despite getting the speeds I want now, I still haven't found an answer to the main question: WHAT IS WRONG WITH THE XB6??!?!?!?!?!?! Clearly there is something going on, but no one seems to know!
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I have had Gigabit internet for a month and a half now and have gotten decent speeds for about two weeks of that time. All the speeds I mention below are over a wired connection.
When I first got the Gigabit service, speeds were great for about a day. Then the problems started. I couldn't get consistent speeds. Sometimes it would be 600-700Mbps, other times it would be between 50 and 400Mbps. I worked with Comcast support for over a week - the came onsite twice, changed some outdoor cables, replaced the modem and suggested I also replace my switch and cables. I was using a Mikrotik router and a Cisco Gigabit switch behind it, all connected to the XB6 modem upstream by a ~50ft Cat5e cable. I figured, sure, why not, and picked up a 50ft Cat6a cable and a new unmanaged Gigabit switch, just to be sure there weren't any issues with my setup. I should mention my previous setup - I had the comcast Gateway in Bridge mode with the Mikrotik acting as the router with the WAN IP assigned to it. It was also handling DHCP, so there weren't any double NAT issues. Yet, despite this device being and enterprise class device less than a year old, it was not getting speeds anywhere close to what it should have been.
Anyway, after the new cables and new switch, I had problems with all that new equipment for about two more days, then after a final call to support, who promised to continue investigating, all the problems just magically stopped and I started getting 890-1010Mbps all day, every day. This lasted until about a week ago. Now I am back to 200-400Mbps speeds.
Nothing else has changed in my setup, so it HAS to be on the Comcast side, though when they sent someone out again, they insisted their testing equipment was getting 900+.
I am in the IT field, so I am no dummy and I know what I am looking at. I am starting to think that there is an issue with the XB6 modem due to some really strange behavior when I connect/disconnect devices from the modem or switch. I have tried connecting directly connecting my laptop to the second port (via 3ft Cat6) while the switch is still connected and I get really bad speeds, like 50Mbps bad. I have also tried connecting directly to the 50ft Cat6 cable and also get worse speed than when connected via the switch. Almost seems like the modem has some kind of ARP issue when you start changing hardware around.
I am at the point where I am considering purchasing another DOCSIS 3.1 modem to see if it helps. Comcast insists there's a problem on my end. Oh yeah? Then why was it working fine for a few weeks, then all of a sudden it went back to the poor performance I had before something was magically corrected by Comcast? Let me know if anyone has had similar issues and what your resolution was. I am too busy to dump a lot of time into testing this, but trust me, the IT side of me really wants to find what the heck is going on. It's just hard when you have no visibility into Comcast's network.
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