STP Blocks a Redundant Link the Team Expected to Use
Two Aruba 2930F switches, SW-A and SW-B, are connected by two Gigabit links: Gi1/1 and Gi1/2. Both run RSTP with default priorities. The team expected both links to carry traffic for load sharing, but monitoring shows Gi1/2 on SW-B is in a blocking/discarding state while Gi1/1 forwards. The switches also connect to an upstream core via a single link each. A junior engineer suggests lowering SW-B's bridge priority to make it 'more active.' Traffic volume is modest and there is no immediate performance problem, but the team wants to understand the design.
1. Explain why one of the two parallel links is blocking and why lowering SW-B's bridge priority will not achieve load sharing.
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RSTP builds a loop-free tree. Two switches joined by two parallel links form a loop, so exactly one of the two links must be in a discarding role (the alternate/backup port). Bridge priority affects which switch becomes root and which ports are chosen as root/designated, but it does not turn a physical loop into an active-active path. Lowering SW-B's priority could make SW-B the root, which might change which of the two links forwards, but one will still block. STP is not a load-balancing mechanism.
To use both links, replace the parallel STP links with a Link Aggregation (LACP) bundle so the pair is treated as one logical link. That is the correct tool for parallel-link capacity.
2. Describe the configuration change that would let both physical links carry traffic, and state one requirement that must be true on both ends.
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Bundle the two ports into a LAG using LACP on both switches:
trunk 1/1-1/2 lacp
(or the equivalent interface lag syntax on ArubaOS-CX). Requirements: both ends must agree on the aggregation (same LACP mode/keys, matching speed and duplex, and consistent VLAN membership). If one side is static and the other LACP, or VLAN tagging differs, the bundle will not come up correctly. Once bundled, STP sees a single logical link, so no port blocks and traffic is distributed across members by hash.