This topic explains how Spanning Tree operates in switch stacks, including the behavior of bridge IDs, stack root election, and reconvergence scenarios when switches join or leave the stack or network.
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A switch stack operating in PVST+ or rapid PVST+ mode appears as a single Spanning Tree node to the rest of the network, and all stack members use the same bridge ID, which is derived from the MAC address of the active stack
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When a new switch joins the stack, it adopts the active stack bridge ID; if it has the lowest ID and equal root path cost, it becomes the stack root.
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When a stack member leaves, Spanning Tree reconvergence occurs, and the remaining member with the lowest stack port ID becomes the stack root.
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Spanning Tree reconvergence may also occur if a neighboring switch external to the stack fails or is powered down, or if a new external switch is added to the network.
Spanning Tree and switch stack reference information
Spanning tree operation in switch stacks involves bridge ID assignment, stack root election, and reconvergence processes during topology changes.
Spanning Tree behavior in switch stacks
If a new switch with the lowest ID joins an existing stack, it becomes the stack root if root path costs are equal. If a stack member leaves, the member with the lowest stack port ID becomes the new root.