This sounds like it's related to what Cisco calls the "native" VLAN on a trunk.

When you configure a switch port for 802.1q tagging (which Cisco boxes still 
call trunking) you can specify which VLANs are allowed on the trunk (default is 
all) and what the "native" VLAN for the trunk should be (default is 1).

Any untagged frames entering the switch from this trunk are assumed to belong 
to 
the "native" VLAN and will be forwarded accordingly to other ports which are 
members of that VLAN.  Frames being written out that trunk port which came from 
the "native" VLAN are sent untagged (all others are tagged with the VLAN number 
that they came from).


I think Solaris attempts to do something similar.  You can attach ce0 to a 
switch port configured for trunking, then plumb ce0, ce123000, and ce456000. 
ce0 sends untagged frames onto the trunk, which the Cisco switch would forward 
into whatever VLAN is set as the "native" VLAN for the trunk.  ce123000 and 
ce456000 would send tagged frames for VLANs 12 3and 456 respectively.

However, last time I tested this in a lab, it did not work as expected.  IIRC, 
I 
never saw any traffic coming in on ce0 even when I sent traffic from another 
system hanging off the same switch whose port was configured as a member of the 
"native" VLAN...

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