This read-only string returns MPLS label-stack information, including label, exp-bits, s-bit, and TTL values for the entire stack, that was encoded in the ICMP error message returned by this hop; it comes back as a null string if the responding router does not support ICMP MPLS extensions. An admin examines it to see whether this hop is an MPLS label-switched router and, if so, what label the traceroute probe was carrying at that point. For example, a populated csOrionTraceRouteResultMplsLabelStackInfo at an intermediate hop confirms the path traverses an MPLS core rather than staying purely IP-routed at that point.