geez guys, I've already said: Cisco EPC 3925
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Looks like one of those "ISP" modem/routers.
They are often crippled in some way, I would not be surprised if that was your problem.
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That might be true, it does not have many knobs, although having ECN on auto and not forced enable seems more useful/logical to me in the first place.

/LE: talking about knobs, the only one I could presume it might do something is "SPI Firewall Protection" where SPI means *stateful packet inspection*, and it just looks like it's at fault in this case. So Cisco EPC 3925 blocks Explicit Congestion Notification (ECN) via its integrated "SPI Firewall Protection" which can be disabled via the web admin interface: 192.168.100.1 -> login -> security -> firewall -> Disable -> Save Settings. The web interface can be accessed even if one can not access the internet.

Regarding the default Linux behaviour I still believe ECN=2 it's saner.
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