This read-only value reports the interface ID at this node's ingress port for the corresponding multipoint circuit's leaf, identifying exactly which physical/logical interface traffic for this leaf entered the switch on. In a multipoint (point-to-multipoint) circuit topology spanning several switch nodes, this pinpoints the local ingress point for one specific branch of the tree, which is essential for tracing the physical path a given leaf's traffic takes through this particular node. An engineer mapping the full path of a misbehaving leaf in a multipoint video-distribution circuit would use cktLeafSrcIfIndex to identify exactly which port on this node the leaf's traffic entered through.