MTU on IPv6 over IPv4 tunnel [XMG3927-B50A]

Hi,

After enabling '6RD', i.e. an IPv6 over IPv4 tunnel, basic IPv6 connectivity works, but I'm experiencing significant timeouts with a subset of sites. This is perhaps best illustrated by a large number of failures in the 'Other IPv6 Sites' tab on https://test-ipv6.com/

I initially suspected a problem with Path MTU Discovery, but I've confirmed that is working as intended, e.g. using http://icmpcheckv6.popcount.org/

Varying the MTU manually, I discovered that the problem appears to be with the tunnel MTU setting. I have an MTU on my IPv4 ISP connection of 1500; the tunnel appears to default to an MTU of 1480. If I lower the MTU to 1412, everything works perfectly - no problems browsing sites and no test failures anywhere.

I think an MTU setting is needed in the 6RD section of the connection settings - unfortunately I don't have the luxury of being able to set it manually on all my networked devices (nor would it be particularly efficient).

Thanks!

Model: XMG3927-B50A
Firmware: V5.17(ABMT.6)C0