IPv6 seems ok: ping ok, but TCP fails

[Celotoise]

Hi,

I'm using a draytek 2925.

The he tunnel ipv6 seems ok : ping both ways are working.

But web TCP sessions fails.

Any help for fix or troubleshooting  is welcome.

[cholzhauer]

Sounds like an MTU mismatch

[Celotoise]

tested multiple IPV6 MTU (1300,1442,1472..) on the windows 10  : no success

Any possibility to monitor or some stat on the traffic from HE point a view ?

[Celotoise]

More info:

from google chrome: http ok but when switching to https timeout during the tls session

Current MTU on HE is 1460

[alerinaldi]

Hello,
this is a thing that made me struggling for many hours. Finally, I discovered this is a quite well-known Broadcom chipset issue:

https://ttlexpired.co.uk/2016/02/12/ipv6-tunnel-and-failing-tcp-sessions/

If you have full control of your router, you can issue those commands to disable flow cache:

fcctl disable
fcctl config --mcast 0

Maybe your router has some GUI options to toggle those settings, too.

[tullygray]

Thanks alerinaldi, this works for me.