Specifies a set of VLAN ids for which frames shall be
forwarded without a tag although ethIfTransmittedFrameType
is set to `vlanTagged(2)'. The setting of this variable has
no meaning if ethIfTransmittedFrameType is set to
`untagged(3)', or if ethFwdVLANMode is set to
`transparent(1)'.
The value of ethFwdVLANMode specifies if the customer or
service VLAN ids are interpreted.