When I create a 6in4/6to4 tunnel, if I leave the MTU option blank, it would be set to 1280, which is the smallest possible value of an IPv6 link.
When I use iproute2 tool to create the tunnel on various Linux distros, it will detect MTU automatically (PMTU discovery). For example, if I have an IPv4 over PPPoE link, whose MTU is typically 1492 (or 1480), it will set the tunnel's MTU to 1472 (or 1460).
Can we take advantage of this?
3.2.1. Static Tunnel MTU
A node using static tunnel MTU treats the tunnel interface as having a fixed-interface MTU. By default, the MTU MUST be between 1280 and 1480 bytes (inclusive), but it SHOULD be 1280 bytes. If the default is not 1280 bytes, the implementation MUST have a configuration knob that can be used to change the MTU value.
Perhaps I'm mistaken; but the current paradigm is according to the RFCs. There is a dynamic config defined in the same document, though, but it's noted that only:
Encapsulators that have a large number of tunnels may choose between
dynamic versus static tunnel MTUs on a per-tunnel endpoint basis. In
cases where the number of tunnels that any one node is using is
large, it is helpful to observe that this state information can be
cached and discarded when not in use.
~ From - 3.2.2. Dynamic Tunnel MTU