This is the source IP address of this dynamically-signaled encap tunnel, inheriting the tunnel-group's address if unspecified, and per the description immutable once the tunnel entry is created -- even though the tunnel's path itself can still change via re-signaling. An admin notes that while the LSP's route can shift dynamically, this source address configuration point cannot, so correcting a wrong source still requires tunnel recreation despite the tunnel otherwise being 'dynamic'. For example, a dynamic tunnel with an incorrectly inherited source address will keep using that wrong source through every future re-signal until the tunnel entry itself is deleted and rebuilt.