In this paper, we investigate a novel problem for dimensioning optical networks with digital subcarrier-based point-to-multipoint (P2MP) coherent transceivers, which we refer to as tree-determination, routing, and spectrum assignment (TRSA). Given a physical optical fiber network with P2MP transceivers and a target IP topology with associated capacity requirements, the TRSA problem involves partitioning capacity demands into logical hub–spoke trees and associated transceiver allocation, along with light-tree routing and spectrum assignment. We propose a heuristic algorithm to solve the TRSA problem with and without fault tolerance for arbitrary network topologies. The algorithms are used to evaluate potential cost benefits of P2MP transceivers for different IP densities and loads in a case study on a metro-network reference scenario. Results indicate that P2MP equipment can bring significant transceiver cost and equipment savings in comparison to point-to-point solutions, although such savings come at a trade-off with increased spectrum occupation.