STP 10: Spanning Tree Troubleshooting Capstone

Bundle lab · $29.99

Advanced Rapid-PVST+ troubleshooting on a 3-switch triangle with a real Layer-2 loop. Two deliberate faults are seeded: an unintended root bridge wins VLAN 100 due to a mis-set/default priority, and PortFast/BPDU Guard are mistakenly applied on an inter-switch trunk. Two Alpine hosts in VLAN 100 verify user impact. Your job: use show commands to diagnose, then restore the correct root and remove edge features from the trunk while preserving them on access ports.

Learning objectives

• Identify the operational root bridge and spanning-tree port roles for VLAN 100 using show spanning-tree outputs.
• Restore deterministic root control by setting an explicit bridge priority so SW1 is the root for VLAN 100.
• Detect and remove PortFast/BPDU Guard from an inter-switch trunk while keeping them on access ports only.
• Confirm Rapid-PVST+ mode is active and that a single port blocks to break the physical loop.
• Validate end-to-end host reachability in VLAN 100 after remediation and interpret why the path changed.

Troubleshooting focus

• If hosts in the same VLAN have intermittent or asymmetric reachability, verify who the root bridge is (priority and MAC) and compare it to design intent.
• On every inter-switch link, confirm the interface is a trunk and not treated as an edge: check edge/PortFast flags and any guards that could err-disable the port on BPDU receipt.
• Look for a link reduction (an intended triangle showing as a V) caused by an err-disabled trunk; confirm the reason and recover cleanly.
• Ensure access ports are the only place PortFast/BPDU Guard is applied; verify the correct VLAN is assigned to each access port.
• Review the spanning-tree summary and per-VLAN detail outputs for consistent Rapid-PVST+ operation, role/state convergence, and a single blocking port in the triangle.