This is the tunnel index bundled into a notification to identify which transit LSP row (one being switched through this node, not originated or terminated here) the alert refers to. Since a transit table can hold many LSPs owned by unrelated head-ends, this index is the only way for the receiving NMS to map a generic 'transit tunnel event' trap to the exact row it should investigate. For example, an alert carrying transit tunnel index 213 tells the operator to check row 213 of the transit tunnel table on this specific node, not the head-end's own tunnel entry.