The value of vRtrIgmpGrpIfMcacPreRsvdMandBW specifies the bandwidth
pre-reserved for all the mandatory channels on all SAPs belonging
to a given group-interface in kilo-bits per second(kbps).
If the value of vRtrIgmpGrpIfMcacUnconstrainedBW is '0', no mandatory
channels are allowed. If the value of vRtrIgmpGrpIfMcacUnconstrainedBW
is '-1', then all mandatory and optional channels are allowed.
If the value of vRtrIgmpGrpIfMcacPreRsvdMandBW is equal to the value of
vRtrIgmpGrpIfMcacUnconstrainedBW, then all the unconstrained bandwidth
on all SAPs belonging to a given group-interface is allocated to
mandatory channels configured through multicast CAC policy on that
group-interface and no optional groups (channels) are allowed.
The value of vRtrIgmpGrpIfMcacPreRsvdMandBW should always be less than
or equal to that of vRtrIgmpGrpIfMcacUnconstrainedBW. An attempt to set
the value of vRtrIgmpGrpIfMcacPreRsvdMandBW greater than that of
vRtrIgmpGrpIfMcacUnconstrainedBW will result in inconsistent value error.