This carries the numeric tunnel index of an encap (ingress-side) tunnel named inside an SNMP trap the device has just raised, delivered as part of the trap payload. Because this object has accessible-for-notify access rather than read-only, you won't find a current value by polling it directly — it only exists as data riding inside the specific SNMP trap that triggered it, so a trap receiver has to capture it at the moment the notification fires. A trap fired for a tunnel event might carry an index value of 473, letting the receiving NMS map the alert back to a specific tunnel table entry.