This represents the source tunnel ID for this gmpls static ingress co-routed tunnel, a numeric identifier the head-end uses (together with the LSP ID) to uniquely distinguish this tunnel instance in the tunnel-identification scheme shared with the far end. An admin uses this value paired with the destination tunnel ID to confirm the head-end and tail-end devices agree on the same tunnel identity, which matters for path validation and for correlating with any signaling-style diagnostics run against this static circuit. For example, a source tunnel ID of 100 must match what the tail-end device expects as the originating tunnel identifier for this same co-routed circuit.