> How is this handled ?
 >
 > The difference I can see if that, with fddi, we know the max len; for IP and
 > for anything else. In our case, the driver needs to inform if_vlan whenever
 > it can do 1522 or only 1518. Not all ethernet boards can do 1522.

ISL is another "perfectly valid" encapsulation that is useful in some
legacy environments; I have some bit-rotted code that does it.

One of ISL's nasty details is the 30 byte encapsulation header/trailer
enclosing an entire L2 packet.  You need 1548 bytes packets from your
ethernet to avoid an MTU reduction.