When the Multiprotocol Label Switching (MPLS) TimeToLive (TTL) field of a packet is set to 0, the packet is discarded.

The TTL field is an 8bit field in the MPLS label that is used to control the propagation of packets through an MPLS network. Thus the MPLS TTL field is similar to the TTL field in an IP header. When an IP packet enters an MPLS network, the ingress router decrements the IP TTL value by 1 and copies that value to the MPLS TTL field. Each MPLS router along the path decrements the MPLS TTL field by 1. When the packet reaches the egress router, the MPLS TTL value is decremented by 1 and copied to the IP TTL field.
If the MPLS TTL field of a packet is set to 0, MPLS TTL propagation is more likely to be enabled than disabled. When MPLS TTL propagation is disabled, the MPLS TTL field is set to 255 and decrements as the packet passes through the MPLS network. When the packet reaches the egress router, the MPLS TTL value is not copied to the IP TTL field. By default, MPLS TTL propagation is enabled, but you can disable it by issuing the no mpls ip propagate-ttl command.
The MPLS TTL field does not indicate whether an MPLS label is the last label in the stack. The BottomofStack field, sometimes called the S field or Stack bit, is a 1bit field that indicates whether the label is the last MPLS label in a packet. A BottomofStack field set to 0 indicates that one or more MPLS labels follow this label. A BottomofStack field set to 1 indicates that this label is the last label in the stack.
The MPLS TTL field does not indicate whether a packet has a low priority. Cisco routers use the 3bit Traffic Class (TC) field in the MPLS label to carry the IP precedence value, which is used to classify and prioritize network traffic. The TC field was formerly designated as the Experimental (EXP) field in Request for Comments (RFC) 3032. However, RFC 3032 did not officially designate the use of the EXP field, so some nonCisco routers use this field for other purposes. RFC5462 officially renames the EXP field as the TC field and designates it to carry traffic class information, such as IP precedence values. Lowpriority traffic might be assigned an IP precedence value of 0, and highpriority traffic might be assigned an IP precedence value of 7.