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Guide

How to diagnose DHCP Snooping

This BAOI guide provides a structured method for diagnosing dhcp snooping without multiplying unnecessary changes.

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

This BAOI guide provides a structured method for diagnosing dhcp snooping 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

DHCP Snooping problems usually come from an incorrect trusted/uplink port, missing bindings, VLAN scope mismatch or rate limiting. Validate the switch decision path before disabling the protection.

Step-by-step checks

  1. Confirm DHCP Snooping is enabled on the affected VLAN and not only globally.
  2. Verify the legitimate DHCP-server/uplink interface is trusted while access ports remain untrusted.
  3. Inspect the binding table for the client MAC, VLAN, IP and lease information.
  4. Check dropped DHCP packets, option 82 behavior and rate-limit violations on the affected access port.

Useful verification commands

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

show ip dhcp snooping
show ip dhcp snooping binding

How to validate the fix

Renew the client lease from an affected access port and confirm Discover, Offer, Request and ACK complete without a Snooping drop while rogue-server replies remain blocked.

Evidence to keep

Record the client MAC/VLAN, switch port, trusted uplink, binding entry and relevant counter/log changes before and after the test.

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

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