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How to test a VLAN

A structured approach avoids looking for the way in when the VLAN does not already reach it.

TopicIT Networking →
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& VLAN Network Intermediate. 15 min.

A structured approach avoids looking for the way in when the VLAN does not already reach it.

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

    List VLANs.

    Note those who have to cross the city.

  2. 2

    Compare the two ports.

    Check mode, allowed VLANs, and native/PVID.

  3. 3

    Observe MAC.

    Check the learning on the VLAN concerned.

  4. 4

    Test gradually.

    Use a VLAN known as a reference.

À retenir

  • Both ends must be consistent.
  • A single functional VLAN does not prove that the complete trunk is correct.
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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.
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