>>	ifp->if_mtu is not initialized in arc_ifattach().  is it intentional?
>	openbsd has the following line in arc_ifattach().
>>        ifp->if_mtu = (ifp->if_flags & IFF_LINK0 ? arc_phdsmtu : ARCMTU);

from man4.amiga/bah.4:

     With the IFF_LINK0 flag cleared, IP and ARP encoding is done according to
     the deprecated, but popular among Amiga users, RFC 1051 encoding (that
     is, with simple header, packet type 240 / 241), and the MTU is 507.

     With the IFF_LINK0 flag set, IP/ARP/RARP encoding is done according to
     RFC 1201 (that is, with Packet Header Definition Standard header and
     packet type 212/213). The MTU is normally 1500.

     When switching between the two modes, do a
     ifconfig interfacename down up to switch the mtu.

	i don't see code to change IFF_LINK0 on "down up" processing.

itojun