Troubleshooting VLAN and Trunk Issues

VLAN and trunking issues can cause communication failures between devices in different VLANs. Below is a step-by-step guide to diagnose and resolve VLAN and trunking problems.

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1. Check VLAN Configuration

Issue: Devices in the same VLAN cannot communicate.

Solution:

• Verify the VLAN exists on the switch:bashCopyEditshow vlan brief
• If the VLAN is missing, create it:bashCopyEditconfigure terminal vlan 10 name Sales exit
• Ensure the ports are assigned to the correct VLAN:bashCopyEditshow interfaces GigabitEthernet0/1 switchport
  • If needed, assign the correct VLAN:bashCopyEditinterface GigabitEthernet0/1 switchport mode access switchport access vlan 10
• If the device still cannot communicate, check if the VLAN is allowed on the trunk.

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2. Verify Trunk Links

Issue: Devices in different VLANs cannot communicate across switches.

Solution:

• Check if the port is in trunk mode:bashCopyEditshow interfaces trunk
  • If the port is missing, it is not operating as a trunk.
• Enable trunk mode manually:bashCopyEditinterface GigabitEthernet0/1 switchport mode trunk
• Check if VLANs are allowed on the trunk:bashCopyEditshow interfaces trunk
  • If VLANs are missing, allow them manually:bashCopyEditinterface GigabitEthernet0/1 switchport trunk allowed vlan 10,20,30

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3. Check Native VLAN Mismatch

Issue: Trunk is up, but VLAN traffic is not passing correctly.

Solution:

• Verify native VLANs on both ends of the trunk:bashCopyEditshow interfaces trunk
  • If one switch has VLAN 1 as native and another has VLAN 99, packets may be dropped.
• Fix the mismatch by setting the same native VLAN on both switches:bashCopyEditinterface GigabitEthernet0/1 switchport trunk native vlan 99

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4. Check Spanning Tree (STP) Blocking

Issue: Some VLANs work, others don’t.

Solution:

• Check if STP is blocking the trunk port:bashCopyEditshow spanning-tree interface GigabitEthernet0/1
• If a port is in blocking mode, try:bashCopyEditshow spanning-tree vlan 10
  • If STP is incorrectly blocking the trunk, consider changing the STP priority:bashCopyEditspanning-tree vlan 10 priority 4096
  • If needed, manually enable the port:bashCopyEditinterface GigabitEthernet0/1 no shutdown

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5. Check Inter-VLAN Routing Issues

Issue: Devices in different VLANs cannot communicate.

Solution:

• If VLANs need to communicate, a router or Layer 3 switch is required.
• Verify if SVIs (Switched Virtual Interfaces) are configured:bashCopyEditshow ip interface brief
• If missing, create an SVI for each VLAN:bashCopyEditinterface vlan 10 ip address 192.168.10.1 255.255.255.0 no shutdown
• Ensure the default gateway is set on end devices:
  • If VLAN 10 uses 192.168.10.1, set this as the gateway.

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6. Check VTP Configuration

Issue: VLANs are not propagating between switches.

Solution:

• Verify VTP mode:bashCopyEditshow vtp status
  • Ensure one switch is in VTP Server mode and others are in VTP Client mode.
• If VLANs are not syncing, try resetting the VTP revision number:bashCopyEditvtp mode transparent vtp mode client

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7. Check for DTP Issues (If Trunk is Not Forming)

Issue: Trunk mode is not working.

Solution:

• Verify the DTP mode on both ends:bashCopyEditshow interfaces switchport
• If both ports are dynamic auto, no trunk will form. Change one side to dynamic desirable:bashCopyEditinterface GigabitEthernet0/1 switchport mode dynamic desirable
• If connecting to a non-Cisco switch, disable DTP:bashCopyEditswitchport mode trunk switchport nonegotiate

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8. Restart the Trunking Process (Last Resort)

If all configurations are correct but the VLAN is still not passing traffic, restart the trunk port:

interface GigabitEthernet0/1
  shutdown
  no shutdown

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Summary of Key Troubleshooting Commands

Command	Purpose
show vlan brief	Checks if VLANs exist on the switch.
show interfaces trunk	Verifies which ports are in trunk mode.
show interfaces switchport	Displays VLAN and trunk settings on an interface.
show spanning-tree interface Gi0/1	Checks if STP is blocking the port.
show vtp status	Confirms VTP domain, mode, and VLAN propagation.
show ip interface brief	Verifies if SVI (inter-VLAN routing) is configured.
show cdp neighbors	Confirms if the switch is connected to the expected devices.

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Best Practices to Avoid VLAN and Trunk Issues

✅ Manually configure trunk ports (switchport mode trunk).
✅ Disable DTP (switchport nonegotiate) unless auto-negotiation is needed.
✅ Ensure native VLANs match on both ends of a trunk.
✅ Use show interfaces trunk to verify allowed VLANs.
✅ Check STP (show spanning-tree) to prevent VLAN blocking.