If I connect the default LAN interface to a VLAN-capable switch, will the LAN interface tag packets with (default) VLAN id 1, or send untagged packets?
In reverse, will LAN interface drop packets tagged with VLAN id 1 or accept them?
VLAN1 will be untagged by default.
If you then TAG packets on a port for VLAN1, then the LAN interface should accept it.