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Guide

How to find an STP loop on a network

This BAOI guide provides a structured method for finding a loop on a network without multiplying unnecessary changes.

TopicIT Networking →
⌚ About 2 min read
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& Switching Network Intermediate. 15-30 min

This BAOI guide provides a structured method for finding a loop on a network without multiplying unnecessary changes.

Avant de commencer : adaptez toujours les commandes et manipulations à votre environnement. Sur un système de production, prévoyez une backup ou un retour arrière lorsque l’action peut modifier la configuration.

Étapes à suivre

  1. 1

    Set perimeter

    Identify the equipment, service, affected users and start time.

  2. 2

    Collect Elements

    Record the exact messages, logs and useful settings.

  3. 3

    Test methodically

    Start with the basic dependencies before the application components.

  4. 4

    Apply correction

    Change only the identified parameter or component.

  5. 5

    Validate

    Rewrite the full scenario and document the result.

À retenir

  • Keep the initial values for a backwards.
  • Avoid multiple simultaneous changes.
  • Compare with a functional configuration when possible.
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Technical deep dive

Layer 2: trunk, STP, LACP and DHCP Snooping require different evidence

Technical checkpoints

  • A trunk must allow the VLAN and both sides must agree on native/untagged behavior.
  • STP intentionally blocks paths to prevent loops; frequent topology changes should be correlated with ports.
  • LACP requires consistent member speed/duplex/VLAN and a recognized partner; a physical link can be up but outside the bundle.
  • DHCP Snooping should trust ports toward DHCP servers/relays, not client-facing ports.

Layer-2 reading

Compare both ends of the link and counters/states rather than only the port LED.

show interfaces trunk
show spanning-tree
show etherchannel summary
show ip dhcp snooping

Topic-specific pitfalls

  • Forcing a port forwarding or disabling STP for testing can create a real network loop.
  • Trusting all DHCP Snooping ports defeats the protection.

How to validate

  • The VLAN traverses expected ports, with no native mismatch or abnormal topology changes.
  • All expected LACP members are bundled and DHCP leases traverse only trusted ports.

Operational diagnosis

An STP loop investigation should correlate topology changes, MAC-address movement and interface utilization. Avoid randomly shutting ports before identifying the loop domain.

Step-by-step checks

  1. Identify the VLANs experiencing broadcast storms, packet loss or rapidly changing MAC locations.
  2. Inspect STP root identity, port roles and recent topology-change counters on the relevant switches.
  3. Look for MAC flapping between two or more ports and abnormally high broadcast/multicast traffic.
  4. Trace the physical or logical path and verify edge protections such as BPDU Guard where appropriate.

Useful verification commands

Run only the commands that match the platform you are troubleshooting and capture the output before making a configuration change.

show spanning-tree
show mac address-table dynamic

How to validate the fix

After isolating the faulty link or configuration, topology-change frequency and MAC flapping should stop, utilization should normalize and the intended STP root/port roles should remain stable.

Evidence to keep

Preserve STP topology outputs, MAC-move logs and interface counters so the correction can be tied to the actual loop path.

Related BAOI resources: IT tools · procedures · IT dictionary.

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