Debugging intermittent Comcast, part 4: Broadband diagnostics

There’s a lot you can potentially do to boost your WAN’s downstream and upload speeds, as well as to minimize its latency. But only if your service provider lets you.

This is my final post in this series. Really. I promise! Like I’ve said before, I’ve learned a lot in the near-year that started last October, when an inadvertent cable cut led to chronic broadband and TV service outages. Four blog posts’ worth, apparently. But I’ll wrap up today. Really. I promise!

At the conclusion of last week’s third post:

I’d been seduced by the Sirens’ Song of the Comcast technician, who, after swapping out some hardware, had confidently proclaimed “wait until you see how fast your Internet access will be now!” Unfortunately, although my connection was now rock-solid reliable, it wasn’t seemingly any faster than before. Why, I wondered? Thereby launching myself down the packet-hole into broadband-land (with apologies to Alice and the White Rabbit for the admittedly lame analogy). Recall upfront that I hadn’t yet done the necessary online research to realize that I was already running near the upper end of my location-determined Comcast-offered speed tier options.

Spectral extension and overlap

DOCSIS 3.1, I learned through my research, is theoretically capable of leveraging wire-based frequencies all the way up to 1,218 Mhz (original plans for further spectrum extensions up to 1,794 Mhz are now instead comprehended in subsequent-gen DOCSIS 4.0). In the process, though, it spectrally overlaps with MoCA 2.5 beginning at 1,125 MHz by default, at least (hold that thought for a dedicated-topic post to come, hopefully in the near future).

Recall, too, that I still had a point of entry (PoE) filter sitting ahead of the cable modem, initially a self-purchased and -installed standalone unit, subsequently integrated within the grounding block supplied by Comcast. In both cases, however, the specified low-pass cutoff frequency was only 1,002 MHz, matching that of the lingering two-way splitter in-between the filter and the cable modem (connection-shared with a CABLEcard receiver, therefore the splitter). And building on the fact that I wasn’t currently running MoCA anyway, I suspected that nobody else in my neighborhood was, either.

Why, then, did all the extra hardware “gifts” the technician had passed on to me at the end of his visit—standalone MoCA filters, two- and three-way splitters, etc.—have 1,002 MHz low-pass cutoffs, too, if DOCSIS 3.1 supposedly stretched to 1,218 Mhz? And what would therefore happen, I wondered, if I were to ditch the outside filter entirely, along with replacing the limited-frequency splitter in the furnace room with a spectrally wider alternative? There’s no better way to find out than to try it and see what happens, right? So that’s what I did.

Here again is the original outside setup:

Here’s my original in-the-furnace-room splitter:

And here are some snapshots of the original dataset, first as a benchmark result from my router.

Followed by a data dump leveraging my cable modem’s convenient second integrated Ethernet port.

Now let’s replace the filter-inclusive grounding block outside with a filter-less one.

And swap out the original furnace room splitter with one that spec’d operation all the way out to 2,450 MHz.

The outcome? Data suggestive of a degraded-performance response to the changes, actually. The benchmark results alternatively might simply reflect test run-to-run variability.

But the higher upstream signal power that the modem is being required to generate in the latter case is, as I first explained in Part 2, suggestive of higher channel noise level that it’s “fighting”.

Perhaps MoCA is operational somewhere in my neighborhood after all? Obviously, I immediately reversed course and returned to the original setup.

Router uniqueness and artificial constraints

The fact that I was “only” getting ~850 Mbps (average, ~890 Mbps peak) download speeds still nagged at me, particularly given that my upload speeds were notably exceeding Comcast’s estimates (43+ Mbps vs. 35 Mbps), and in spite of the fact that I’d best-case only be able to squeeze an additional 50 Mbps or so of downstream bandwidth out of the setup. My cable modem is a somewhat unique Netgear variant, the Nighthawk CM1100, which I’d first mentioned more than seven years back, with more in-depth coverage a year-plus later.

As I mentioned before, the Nighthawk CM1100 “only” implements 1 GbE connectivity, but it integrates two Ethernet ports, thereby (barely) rationalizing NETGEAR marketing’s “multi-gig” product claims.

The intent here is to optionally mate the CM1100 with a more advanced router containing dual Ethernet WAN connections, subsequently leveraging a feature known as “link aggregation”. But, as my research eventually revealed, this particular feature, not to mention the router’s broader 1 GbE (minus protocol overhead) claimed peak performance specs, were for naught “thanks” to Comcast’s unavoidable setting tweaks.

Here’s what Google AI Assistant, aggregating various info bits on the Internet that I’d already come across, spat back at me when I searched on the phrase “Xfinity 1 Gbit plan actually 800 Mbit CM1100”.

The reason your Xfinity 1 Gbps (1,000 Mbps) plan is maxing out at roughly 800 Mbps is because Xfinity explicitly limits the Netgear CM1100 to an 800 Mbps speed tier on their network.

Even though the Netgear CM1100 is a highly capable DOCSIS 3.1 modem with a theoretical multi-gigabit capability, Comcast’s official device compatibility lists cap this specific hardware for plans “Up to 800 Mbps” (previously rated for up to 945 Mbps). When an unapproved or downgraded modem is connected to a higher tier like the 1 Gig plan, Xfinity’s automated provisioning system pushes a default, restricted config file to the modem. This prevents it from pulling your full subscription speed.

Hardware and Network Bottlenecks

Beyond Xfinity’s strict software provisioning profile, your hardware setup likely has several physical bottlenecks preventing you from reaching 1 Gbps:

Lack of a 2.5 Gbps Port: The CM1100 only features standard 1 Gbps (1,000 Mbps) Ethernet ports. Due to standard network overhead (headers, routing metadata, and packet loss), a single 1 Gbps port will max out at roughly 940 Mbps in a real-world wired environment.
Link Aggregation Limits: While the CM1100 supports Link Aggregation (bonding its two 1 Gbps ports together to achieve 2 Gbps), Xfinity generally disables or restricts this feature on customer-owned modems. Furthermore, most consumer Wi-Fi routers do not support WAN Link Aggregation.

How to Fix It

If you want to get the full speed you are paying for, you have three primary paths forward:

Downgrade Your Plan: Call Xfinity and drop your service to the 800 Mbps tier. Your CM1100 is perfectly optimized for this tier, you will save money on your monthly bill, and your real-world internet performance will feel exactly the same.
Upgrade Your Modem: If you want to keep the 1 Gbps speed (or plan to upgrade to 1.2 Gbps in the future), you must purchase a modern DOCSIS 3.1 or DOCSIS 4.0 modem that features a dedicated 2.5 Gbps Ethernet port (such as the Arris Surfboard S33 or Netgear CM2000). This will allow Xfinity to push a “Next-Gen” provisioning profile to your house.
Rent an Xfinity Gateway: You can opt to rent an Xfinity XB7 or XB8 gateway. These come equipped with native 2.5 Gbps LAN ports and are completely optimized for Xfinity’s over-provisioned gigabit speeds.

Unfortunately, as you already saw from the screenshots I shared last time, an “800 Mbps” plan isn’t offered at my particular Comcast service location. I’m also not interested in paying Comcast a monthly rental fee for the “privilege” of using Xfinity Gateway gear I’ll never own free-and-clear.

And regarding the “upgrade your modem” option, I now recall an email I received from the company in late May 2025 with the tantalizing subject line “Replace your equipment to get faster speeds – at no extra cost.” Here’s the body verbiage.

Good news

Bringing customers like you the best in-home WiFi experience is our top priority. That’s why we’re excited to give you faster speeds so your home can continue being everyone’s favorite binge, scroll, share, and stream zone.

Here’s the best part: These faster speeds are included with your current Internet plan at no extra cost.

Next steps

Your current internet equipment can’t deliver these new speeds, so we’ve compiled a list of compatible devices to purchase so that you can take advantage of the fastest speeds available to you. Click below to find a compatible device.

Find compatible devices

Or you can explore our Xfinity Gateway option, which is a modem + WiFi router in one with advanced security designed to deliver the fastest, most reliable coverage on our network.

Take action today so you can start enjoying faster speeds as soon as possible.

Thanks for being with us.

The “no extra cost” angle, perhaps obviously, was specific to the service tier I was paying for. The “replace your equipment” encouragement, on the other hand, was most definitely not “no extra cost”. And nowhere in the email, or the linked website page for that matter, did Comcast happen to mention Google AI Assistant’s tipoff that “when an unapproved or downgraded modem” such as my existing CM1100 “is connected to a higher tier like the 1 Gig plan, Xfinity’s automated provisioning system pushes a default, restricted config file” (known as a bootfile) “to the modem. This prevents it from pulling your full subscription speed”.

The information Google AI’s Assistant is sharing with me seems spot-on, by the way; an over-provisioned “800 Mbit” plan (if it theoretically existed as a service tier option for me) would likely deliver the ~890 Mbps peak download and ~43 Mbps peak upload speeds that I’m now seeing.

I’m more than a little irritated to learn that Comcast’s latest-version provisioning bootfile for CM1100, which downloads every time the modem connects to the network and overrides NETGEAR’s factory-default settings, is disabling my device’s dual-Ethernet facilities. I’m even more irritated to learn that the bootfile now also artificially restricts my modem’s peak speeds, which were previously unhindered.

And don’t get me started on the fact that Comcast wants me to drop several hundred dollars on a brand-new replacement modem with capabilities that the speed tiers available at my location can’t even leverage, discarding my perfectly good existing hardware in the process, just so I can take full advantage of the service I’m paying for.

Thanks but no thanks, Comcast. Thoughts, readers? Sound off in the comments!

—Brian Dipert is the associate editor, as well as a contributing editor, at EDN.

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