The value of tmnxL2tpLnsTuPppDefaultGroupIf specifies the
group interface where the PPP sessions are established
when the authentication server does not specify the group interface.
The empty string specifies that the actual value must be used
of the object tmnxL2tpLnsTgPppDefaultGroupIf.
If the value of tmnxL2tpLnsTuPppDefaultGroupIf is equal to zero, the
value of tmnxL2tpLnsTuPppDefaultService must also be equal to zero.