What is the role of the S field in an MPLS label?

The S field in an MPLS label serves to indicate whether a label is at the bottom of the stack. In Multiprotocol Label Switching (MPLS), labels can be stacked to support hierarchical routing or multiplexing of data paths. When multiple labels are present, the S field informs the MPLS router about the position of a specific label within the stack. If the S field is set, it indicates that this label is the last label to be processed and is the final destination for the packet.

This identification is crucial because it allows routers in the MPLS network to handle packets appropriately, ensuring that the right label is examined at the right stage in the routing process. The functionality of the S field helps maintain the integrity of the MPLS structure, especially when handling complex routing scenarios that involve multiple labels.

In contrast, the other options refer to different aspects of MPLS labels. The S field does not identify whether the label is top-of-stack or mark it for routing; the TTL (Time to Live) value is represented by a distinct field within the MPLS header. Therefore, the role of the S field specifically focuses on designating the bottom-of-stack status of the label.