To configure extended discovery to support the MPLS LDP session between the headend and tailend routers, the engineer must configure a targeted neighbor session. This action allows for the establishment of LDP sessions between routers that are not directly connected, which is necessary for setting up MPLS tunnels that span multiple hops in the network.

References := The concept of targeted LDP sessions is covered in the Implementing and Operating Cisco Service Provider Network Core Technologies (SPCOR) curriculum, which explains how these sessions are used to establish LDP relationships over a non-directly connected interface, facilitating MPLS tunneling for applications like video streaming.