On 2019-03-03 11:31, Mark Tinka wrote: [..] > Across the 6-in-4 tunnel, the tested MTU is 1,232 for IPv6.
IPv6 requires a minimum MTU of 1280. If you cannot transport it, then the transport (the tunnel in this case) needs to handle the fragmentation of packets of 1280 down to whatever does fit in the tunnel. [..] > So the right combination of settings is to have 1,280 bytes on the Mikrotik > and enable "ipv6 tcp adjust-mss 1232" on the Cisco (the latter for my > specific case - yours may vary depending on your IPv4 conditions). Have fun with all your UDP traffic that does not care about your TCP MSS adjustment. You just hid the problem... And a correctly configured MTU is especially going to be fun with "HTTP/3" that is being pushed through, even though the predecessor QUIC does not care about MTU at all... good that it is all in the hands of a company that can fix it themselves ;) Greets, Jeroen
