Here’s a step-by-step VTP trunking protocol troubleshooting checklist you can follow when VLANs aren’t propagating between switches:

August 11, 2025

Here’s a step-by-step VTP trunking protocol troubleshooting checklist you can follow when VLANs aren’t propagating between switches:

Here’s a step-by-step VTP trunking protocol troubleshooting checklist you can follow when VLANs aren’t propagating between switches:

1️⃣ Check the Trunk Link Status

Command:

show interfaces trunk
show interface <int> switchport

Verify:
- Trunk link is up and operating in trunk mode.
- Encapsulation type matches (Dot1Q or ISL – mostly Dot1Q now).
- Both ends have the same allowed VLAN list.

2️⃣ Verify VTP Domain Name

Command:
```
show vtp status
```
Both switches must have the exact same VTP domain name (case-sensitive).
If mismatched → VLANs will not propagate.

3️⃣ Check VTP Mode

Modes:
- Server – can create, modify, and delete VLANs (propagates changes).
- Client – receives VLANs from server, cannot create VLANs locally.
- Transparent – does not participate in VTP advertisements (but forwards them).
Mismatch issue: If both ends are Transparent, no VLAN propagation.

4️⃣ Verify VTP Version

Command:
```
show vtp status
```
Both ends must be using the same VTP version:
- VTPv1
- VTPv2 – supports Token Ring VLANs.
- VTPv3 – supports extended VLANs and authentication.
Mismatched versions can block updates.

5️⃣ Check VTP Pruning

If pruning is enabled, unused VLANs will be pruned off the trunk automatically.
Command:
```
show vtp status
```
If VLANs are missing, check if they are being pruned.

6️⃣ Validate VLAN Existence

Command:
```
show vlan brief
```
The VLAN you want must exist in the server switch for it to propagate.

7️⃣ Check VTP Password

If a password is configured, it must match on all switches in the domain.
Command:
```
show vtp password
```
Incorrect password will prevent VLAN syncing.

8️⃣ VTP Revision Number

Command:
```
show vtp status
```
Higher revision number overwrites VLAN database.
If a new switch with a higher revision number (but empty VLAN database) is connected, it can erase VLANs on the network.
Reset revision number by changing the domain name temporarily and setting it back.

9️⃣ Verify Native VLAN

Native VLAN mismatch can cause trunk issues.
Command:
```
show interfaces trunk
```
Ensure native VLAN is the same on both sides.

🔟 Physical Layer & Cabling

If the trunk isn’t coming up:
- Check fiber/copper cables.
- Ensure the right port speed & duplex.
- Test with a known working cable.

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