This is the tunnel index carried inside a trap/notification payload, identifying which specific encapsulation (head-end) tunnel the event refers to so the NMS can look up the matching row in the encap tunnel table. Without this binding value the notification alone would just say 'a tunnel changed state' with no way to know which of potentially hundreds of head-end tunnels triggered it. For example, a notification carrying index 47 tells the receiving management station to go pull cienaCesMplsEncapTunnelTable row 47 for the full context of what happened.