In an 802.1D Spanning Tree Protocol (STP) domain, network-wide timers—specifically the Hello Time, Forward Delay, and Max Age—are determined and dictated by the root bridge. The root bridge embeds these timer values into the Bridge Protocol Data Units (BPDUs) it sends out. All other switches in the STP domain, including the legacy 802.1D switches, receive these BPDUs and adopt the timer values set by the root. To influence the convergence time for these legacy switches, the timers must be modified at their source, which is the root bridge, typically located at the network core in a Core/Aggregation/Edge topology.

A. the route-only port on the upstream gateway: "Route-only port" is not a standard STP term. Furthermore, modifying timers on any non-root device is ineffective as they are overridden by the root bridge's BPDUs.

B. on the two legacy 802.1D switches at the edge: These switches are not the root bridge. Any local timer configuration on them will be ignored and replaced by the values received in BPDUs from the root bridge.

C. on the Aggregation switches connected to the legacy edge switches: Although these switches form the boundary between RSTP and legacy STP, they are not the root bridge and will simply propagate the timer values received from the root.

1. RUCKUS FastIron Command Reference, 8.0.90 (fi-8090-commandref.pdf): In the chapter for Layer 2 Switching Commands, the descriptions for the spanning-tree forward-time, spanning-tree hello-time, and spanning-tree max-age commands explicitly state: "These parameters are significant only on the root bridge. The root bridge sends BPDUs containing these parameters to the other bridges in the network." (See pages related to Spanning Tree global configuration).

2. IEEE Std 802.1D-2004, "IEEE Standard for Local and Metropolitan Area Networks--Media Access Control (MAC) Bridges": Clause 8.4, "Bridge Protocol Data Units (BPDUs)," specifies that Configuration BPDUs carry the timer values (Message Age, Max Age, Hello Time, Forward Delay) and that these values are originated by the Root Bridge for the Spanning Tree. Non-root bridges use these values from the received BPDUs.

3. Kurose, J. F., & Ross, K. W. (2017). Computer Networking: A Top-Down Approach (7th ed.). Pearson. In the section on Link-Layer Switches, the operation of the Spanning Tree Protocol is described, clarifying that the root bridge is responsible for sending configuration messages that dictate the topology and its parameters, including timers, for the entire switched network.