1. Nokia 7750 SR OS Services Configuration Guide, Release 21.10.R1: In the section describing Spanning Tree, the port roles are defined. The guide clarifies the Backup Port role in the context of RSTP and MSTP. "A backup port provides a redundant path to the segment to which it is connected. A backup port exists when a bridge has two or more connections to the same segment (for example, through a hub)." This inherently means the Designated Port for that segment is on the same bridge. (Reference: Section on "RSTP" and "Port States and Roles").
2. IEEE Std 802.1Q-2018, "Bridges and Bridged Networks": This standard incorporates RSTP. In Clause 13.5, "Port Roles," the Backup Port role is formally defined. It states: "A port’s role is Backup Port if it receives more useful Spanning Tree information from a Designated Port on the same Bridge." This explicitly confirms that the Backup Port and the corresponding Designated Port are on the same switch (Bridge). (Reference: Clause 13.5, page 478).
3. "Spanning Tree Protocol: A deep dive into the 802.1D standard" - University of California, Berkeley, EECS Department Course Material: Course notes for advanced networking often detail RSTP port roles. Lecture materials explain that "A backup port is a port blocked by receiving BPDUs from a designated port on the same switch. This happens when two ports of a switch are connected to the same segment." (Reference: CS168/268 Course Notes on Spanning Tree Protocol).