This read-create InterfaceIndex identifies the ingress transponding interface for one direction of an optical transponder connection, and together with optXpdConnConfigEgressIf forms the index into the dynamic transponding connection table. It tells the admin which physical interface traffic enters on for this cross-connect, which is essential when wiring up 2.5Gb transponder paths. For example, an admin creating a new connection row might set this to the ifIndex of the transponder's client-side port.