Xfinity Gigabit internet losing DNS frequently

[dbwiddis]

This issue has similar symptoms to this thread and this thread and this thread and probably many others.   Symptom:  random disconnections multiple times per day.   Established connections (e.g., video calls) maintain their connection, however, attempts to visit new websites fail with bad DNS resolution.   Speed test from my Orbi router direct-connected to my cable modem still functions and provides high speeds, but it appears to have a hardcoded DNS destination.

Steps I've already taken:

 - called XFinity customer support. Tech visited, checked signal on cables, replaced all connectors and splitter, removed disconnected outlets, etc.  Current setup has cable from external drop straight into house with a single splitter (3.5db drop on 2 outlets), one to my cablemodem and one to my TV.   Additionally because of high downstream signals I installed a 6db attenuator between splitter and cablemodem.

 - Have replaced cable modem to rule it out as an issue.  Problem occurs with both Netgear CM1000 and Motorola MB8600.

Rebooting the cablemodem (either locally with powercycle or remotely via Xfinity app "troubleshooting") temporarily restores connectivity, until it fails again...

Since all the other threads indicate upstream/downstream signal strength or SNR as likely culprits I'll post the numbers below, but I'm pretty confident they're about as good as they can be.

Any advice on further troubleshooting or specific tech support requests I can make to Xfinity?  

Downstream Bonded Channels

Channel Lock Status Modulation Channel ID Freq. (MHz) Pwr (dBmV) SNR (dB) Corrected Uncorrected    1 Locked QAM256 17 567.0 1.0 42.6 0 0    2 Locked QAM256 1 459.0 1.4 43.4 0 0    3 Locked QAM256 2 465.0 1.5 43.3 0 0    4 Locked QAM256 3 471.0 1.4 43.3 0 0    5 Locked QAM256 4 477.0 1.6 43.5 0 0    6 Locked QAM256 5 483.0 1.5 43.1 0 0    7 Locked QAM256 6 489.0 1.8 43.6 0 0    8 Locked QAM256 7 495.0 1.9 43.5 0 0    9 Locked QAM256 8 507.0 2.1 43.5 0 0    10 Locked QAM256 9 513.0 2.0 43.5 0 0    11 Locked QAM256 10 519.0 1.9 43.4 0 0    12 Locked QAM256 11 525.0 1.8 43.3 0 0    13 Locked QAM256 12 531.0 1.9 43.2 0 0    14 Locked QAM256 13 543.0 1.4 43.2 0 0    15 Locked QAM256 14 549.0 1.4 42.7 0 0    16 Locked QAM256 15 555.0 1.3 42.9 0 0    17 Locked QAM256 16 561.0 1.2 42.8 0 0    18 Locked QAM256 18 573.0 1.0 42.6 0 0    19 Locked QAM256 19 579.0 1.0 42.1 0 0    20 Locked QAM256 20 585.0 1.0 42.3 0 0    21 Locked QAM256 21 591.0 0.9 42.1 0 0    22 Locked QAM256 22 597.0 0.8 42.1 0 0    23 Locked QAM256 23 603.0 0.8 42.0 0 0    24 Locked QAM256 24 609.0 0.5 42.1 0 0    25 Locked QAM256 25 615.0 0.2 41.9 0 0    26 Locked QAM256 26 621.0 0.2 41.7 0 0    27 Locked QAM256 27 627.0 -0.2 41.6 0 0    28 Locked QAM256 28 633.0 -0.3 41.5 1 0    29 Locked QAM256 29 639.0 -0.6 41.4 0 0    30 Locked QAM256 30 645.0 -0.7 41.3 0 0    31 Locked QAM256 31 651.0 -0.8 41.2 0 0    32 Locked QAM256 32 657.0 -0.9 41.2 0 0    33 Locked OFDM PLC 159 722.0 2.6 41.0 0 0

Upstream Bonded Channels

Channel Lock Status Channel Type Channel ID Symb. Rate (Ksym/sec) Freq. (MHz) Pwr (dBmV)    1 Locked SC-QAM 1 5120 30.1 43.3    2 Locked SC-QAM 2 5120 36.5 42.8    3 Locked SC-QAM 3 5120 17.3 42.3    4 Locked SC-QAM 4 5120 23.7 42.3

[dbwiddis]

Issues recurring today.  Seem more prevalent in early morning (8am to 10am window).   Still have good connection if I set DNS on my own machine, but DNS via router/cablemodem to Comcast (75.75.75.75 and 75.75.76.76)  is dead and doesn't come back without rebooting cablemodem.

[dbwiddis]

OK, I think I have a theory.  This answer describes how Comcast's DNS resolution works with the "Anycast" redirection.   And this answer in the same thread identifies DNS latency to Comcast DNS as an issue.

I do live in the greater seattle area which includes a high tech population recently relegated to working from home, and I suspect Comcast's local (to Seattle/Bellevue/Redmond) DNS server just can't handle the load, particularly at key times in the day (8-9 when everyone starts working, and around lunch and end of the day when everyone starts surfing news, etc.).

Since I just have a modem and separate router, I have worked around the problem by changing my DNS lookups away from the Xfinity-supplied results.