The unique value which identifies this group-interface of this virtual
router in the Tmnx system. This field provides an identifier for
virtual router interfaces similar to the ifIndex values used in the
mib-2 interfaces table for physical interfaces.
The vRtrPimNgGrpSrcGrpIfIndex values
can be reused for different virtual routers, but are unique within
those entries with the same vRtrID.
The vRtrPimNgGrpSrcGrpIfIndex value for each virtual interface must
remain constant at least from one re-initialization of the system
management processor to the next.