1.2 Gbps service only seeing 400 Mbps speeds

Just for some additional information.

I'm also aware the T25 is not in the support matrix for the 1.2 Gbps service. I didn't check this when I was upsold as my plan was to go with the 900 Mbps service (which it is supported with). Wanting to remedy this, I attempted to call and have them downgrade the service and was told they could not do that without greatly increasing the price of my bill. This makes no sense whatsoever to me hence why I'm trying to make this work with the faster service, knowing I'll be limited to only ~ 900 Mbps anyway due to my equipment. I have heard there could be some issues with this setup however so perhaps I need to call again and talk to yet someone else who will tell me a completely different story, as is the case with every representative you speak with.

... knowing I'll be limited to only ~ 900 Mbps ...

Probably a lot less than that. AFAIK Comcast has never said just how they handle devices on speed tiers for which they are not approved, but users have reported 1/2 subscribed speed or even less. You need to pick one of the modems on https://www.xfinity.com/support/devices/ that is approved for the 1.2 Gbps speed tier, or drop back to a lower one.

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I chatted with another tech support agent last night who ran some diagnostics and reported some errors on the line. I explained the situation regarding the modem being unsupported and he did not seem concerned (not that means anything). Due to the errors, they suggested a tech be sent out. They will be out tomorrow morning.

As I did not want any surprises, I ran a brand new cable line outside to the Xfinity service box on my house directly to the modem, with the rest of the house fully disconnected from the service. I ran various speed tests and again saw the usual 400 - 430 Mbps range. This proves the issue is not within my line distribution inside of the house.

So either the tech will find something outside of my home or it is in fact the modem, which I really hope its not being the agent last night could have saved a tech visit to simply drop me to 900 Mbps, something I've been asking to try since Saturday. Will at least keep this thread updated as I work through the issue as I saw this type of problem reported numerous times in multiple forums but it seems people don't like to report what the underlying issue was once fixed.

Well, tech support came by and ran a test at the modem and everything checked out. He was here for all of 10 minutes and simply blamed the modem for the speed, as the signal looked great according to him.

I then called support yet again to downgrade me to the 900 Mbps service. Long story short, they are charging me $50 more a month for the 900 Mbps service since it removed the promotion I was put on at the end of last week. They would do absolutely nothing to help me. I offered to still pay the same price and just use the 900 Mbps service, I didn't want a discount or anything. I just want to get over 400 Mbps. Again, absolutely nothing they would do. Customer service is such a sham these days, its unfortunate. I guess I'll be exploring the AT&T Fiber that they just ran in my neighborhood in the near future.

The best part is, 900 Mbps service made absolutely no difference. Still maxing out at 400 Mbps directly at the modem. Very unfortunate indeed. In short, for those still following along, give up. It gets no better. I've put 30 hours of time into this now for zero improvement. I'm sure I'll get charged for the tech visit today as well. Good times.

@astiesi​ That and kind of a reality with cable distribution depending on congestion in your area and the infrastructure you are talking to.  A storm wiped out a bunch of infrastructure at a location I have in another state, and now that one is way faster than this location using identical hardware (was slower before).   Similar line conditions at both places.  No uncorrectable errors at either place, and power/SNR roughly the same.

Another truth?  You aren't going to see a 900Mbps download on anything.  Perhaps pulling an .iso from a Rufus mirror near you or something like that.  We're all load balancing and throttling at what you are talking to.  For streaming 4K, you're lucky if you see a 45Mbps connection from a streaming server in really good 4K (~20GB/hour).  They've got other customers too.  You can look at that in real time with other equipment if you don't believe me.

Unless you are trying to host a lot of users all pulling down bandwidth at the same time, I'm not sure what the point of paying for faster service would be.  The upload speed is slow, so you aren't going to run any hosting services from an Xfinity account.  If you need to do that, renting rack space or running VM's in a server farm on an actual backbone is the way to go. 

Fiber is different.  It either works at the speed you pay for, or it does not and the speed is consistent.  Same limitations on the end you are talking to, but your 'speed test' will work better and if you have a lot of users, you'll get the full bandwidth.

I'd still complain though.  There probably IS a problem upstream from you.  Either that, or go with a much cheaper speed tier, cause it doesn't matter anyway.  PLC lock problem with your other modem and speed tier mismatch?  Maybe.  There's no data from Xfinity or a white paper on it from anyone.