Set by the operator to indicate the way the VC shim label
EXP bits are to be determined. The value of outerTunnel(1)
is used where there is an outer tunnel - cpwVcMplsMplsType
is mplsTe or mplsNonTe. Note that in this case there is no
need to mark the VC label with the EXP bits since the VC
label is not visible to the intermediate nodes.
If there is no outer tunnel, specifiedValue(2) indicate
that the value is specified by cpwVcMplsExpBits, and
serviceDependant(3) indicate that the EXP bits are setup
based on a rule specified in the emulated service specific
tables, for example when the EXP bits are a function of
802.1p marking for Ethernet emulated service.