The root bridge is connected to another switch over two independent 10Gbe links. Which link will remain in a forwarding state on both sides?

In Spanning Tree Protocol (STP), when a non-root switch has multiple, equal-cost links to the root bridge, it must select one port as its Root Port to forward traffic and block the others to prevent a loop. The selection is based on the Bridge Protocol Data Units (BPDUs) received from the root bridge. After path cost, the tie-breaker is the lowest Sender (Designated) Bridge ID, which is the same for both links. The next tie-breaker is the lowest Sender Port ID. The root bridge's port with the lowest number will have the lowest Port ID (assuming default priorities). The non-root switch will select its port connected to this link as its Root Port, keeping the link in a forwarding state on both ends.

A. The non-root switch's port number is only used as a final tie-breaker if all preceding criteria, including the Sender Port ID from the root bridge, are identical.

C. Spanning Tree Protocol always prefers lower values for costs, bridge IDs, and port IDs during its election and tie-breaking processes, not higher values.

D. The non-root switch's port number is not the primary tie-breaker in this scenario, and STP prioritizes lower values, not higher ones.

1. RUCKUS FastIron Layer 2 Switching Configuration Guide, 08.0.95, Chapter 13: Spanning Tree Protocols, Section: "How a RUCKUS device selects the root port", Page 338. This section outlines the tie-breaking criteria for Root Port selection, stating: "4. The port connected to the bridge that has the lowest port ID." This confirms the decision is based on the sender's (root bridge's) port ID.

2. RUCKUS ICX 7850 Layer 2 Switching Configuration Guide, 08.0.95, Chapter 10: Spanning Tree Protocol (STP), Section: "Root port selection", Page 239. The guide specifies the selection process: "If the path cost and the designated bridge ID are the same, the port that is attached to the upstream device port with the lowest port ID is selected as the root port." In this scenario, the root bridge is the upstream device.