1. Nokia Official Documentation: Nokia 7450 ESS, 7750 SR, 7950 XRS, and VSR Layer 2 Services and EVPN Guide, Release 22.10.R1, Section: "Spanning Tree Protocol (STP)". The documentation describes how STP, RSTP, and MSTP operate by creating a single active path to a root, which inherently means other direct paths are blocked. The guide states, "The Spanning Tree Protocol (STP) is a Layer 2 protocol that runs on bridges and switches... STP creates a single spanning tree for the entire bridged network, which results in a single active path between any two nodes." This confirms the creation of a single path, which is not necessarily the most optimal one if a more direct link is blocked.
2. Academic Publication (Original Protocol Design): Perlman, R. (1985). "An algorithm for distributed computation of a spanning tree in an extended LAN." ACM SIGCOMM Computer Communication Review, 15(4), 44–53. https://doi.org/10.1145/318951.319004. While the paper details the algorithm for creating a loop-free tree, subsequent analysis and networking pedagogy built upon this work universally acknowledge that the resulting tree does not guarantee shortest paths between all pairs of nodes, only between each node and the root.
3. University Courseware: Kurose, J. F., & Ross, K. W. (2021). Computer Networking: A Top-Down Approach (8th ed.). Pearson. In Chapter 6, "The Link Layer: Links, Access Networks, and LANs," the text explains the operation of spanning tree. It clarifies that the protocol forces traffic to follow the branches of the tree, even if a "shortcut" link (which is blocked by STP) exists, thus leading to suboptimal routing between some pairs of switches. This textbook is standard curriculum in numerous university computer science and engineering programs.