Troubleshooting Dynamic Trunking Protocol (DTP) Issues

February 23, 2025

If DTP is not forming trunks as expected, you need to check for common misconfigurations and conflicts. Below is a systematic troubleshooting guide.

1. Verify the Current Trunking Status

Issue: The trunk is not forming.

Solution:

Use the following command to check if the port is in trunk mode:

show interfaces trunk

If the port is missing from the output, it means the trunk did not form.

Also, check the interface mode with:

show interfaces GigabitEthernet0/1 switchport

Look for Administrative Mode and Operational Mode.
Ensure the port is not in access mode if you expect it to be a trunk.

2. Check the DTP Mode on Both Ends

Issue: Two switches are not forming a trunk.

Solution:

Use:

show running-config interface GigabitEthernet0/1

Verify that both ports are using compatible DTP modes.

Local Mode	Remote Mode	Trunk Formed?
Trunk	Trunk / Dynamic Desirable / Dynamic Auto	✅ Yes
Dynamic Desirable	Trunk / Dynamic Desirable / Dynamic Auto	✅ Yes
Dynamic Auto	Dynamic Auto	❌ No (Both are passive)
Access	Any Mode	❌ No

👉 If both ports are set to dynamic auto, change one to dynamic desirable or trunk:

interface GigabitEthernet0/1
  switchport mode dynamic desirable

3. Check for DTP Being Disabled (`nonegotiate`)

Issue: Trunk is not forming even though the correct mode is set.

Solution:

Check if DTP negotiation is disabled:

show running-config interface GigabitEthernet0/1

If you see switchport nonegotiate, DTP is disabled.

👉 If needed, remove the nonegotiate command:

interface GigabitEthernet0/1
  no switchport nonegotiate

4. Verify VLANs Allowed on the Trunk

Issue: Trunk is formed, but some VLANs are missing.

Solution:

Check which VLANs are allowed on the trunk:

show interfaces trunk

If some VLANs are missing, allow them manually:

interface GigabitEthernet0/1
  switchport trunk allowed vlan add 10,20,30

5. Check Native VLAN Mismatch

Issue: Trunk forms, but communication issues occur between VLANs.

Solution:

Verify the native VLAN on both switches:

show interfaces trunk

If one switch has VLAN 1 as native and another has VLAN 99, packets might be dropped.

👉 To fix, match the native VLAN on both sides:

interface GigabitEthernet0/1
  switchport trunk native vlan 99

6. Check for Trunking with Non-Cisco Devices

Issue: Trunk is not forming with a non-Cisco switch.

Solution:

DTP is Cisco-proprietary and does not work with non-Cisco switches.

Manually set the port to trunk mode and disable DTP:

interface GigabitEthernet0/1
  switchport mode trunk
  switchport nonegotiate

👉 This forces the trunk without DTP negotiation.

7. Restart the Trunking Process (Last Resort)

Issue: All configurations are correct, but the trunk is still not working.

Solution:

Try resetting the trunking interface:

interface GigabitEthernet0/1
  shutdown
  no shutdown

This restarts the trunk and can force re-negotiation.

Summary of Key Troubleshooting Commands

Command	Purpose
`show interfaces trunk`	Checks which ports are operating as trunks.
`show interfaces switchport`	Displays DTP mode, VLAN settings, and negotiation status.
`show running-config interface Gi0/1`	Checks if `nonegotiate` is enabled.
`show vlan brief`	Ensures VLANs are properly assigned.
`show cdp neighbors`	Confirms neighboring Cisco devices.

Best Practices to Avoid DTP Issues

✅ Manually set trunk mode (switchport mode trunk) instead of relying on DTP.
✅ Disable DTP (switchport nonegotiate) to prevent unauthorized trunking.
✅ Ensure Native VLANs match on both ends of the trunk.
✅ Use show interfaces trunk to verify allowed VLANs.

The Lazy Networker