Troubleshooting VTP: Incorrect VLAN Configuration and Solution

By | 9th November 2015

We’ve learned that VLAN is a means of transmitting packets across a switched network via a trunk link, also for traffic from a VLAN to be transmitted across a trunk it has to be allowed access on the trunk.

The command used to do this is the switchport access trunk allowed vlan add {vlan-id }command.

In the topology diagram below, VLAN 20 (Sales dept) and computer PC5 have been added to the network. This is to show that the VLANs allowed on the trunk are 10, 20, and 99.

Incorrect VLAN configuration and Solution.

In this example, computer (PC5) cannot connect to the sales department e-mail server.

To solve this, we will start by verifying the trunk port in S1 using the show interfaces trunk command:

S1#show interface trunk

Port     Mode     Encapsulation   Status     Native vlan

Fa0/1     on          802.1q                 trunking  99

Fa0/3     on          802.1q                trunking  99

Port   vlans allowed on trunk

Fa0/1  10, 99

Fa0/3  10, 99


The output of the command above shows that the F0/3 interface on switch S1 reveals that interfaces F0/1 and F0/3 only allow VLANs 10 and 99.

Using the same show interfaces trunk command on S3:

S3#show interface trunk

Port      Mode     Encapsulation   Status     Native vlan

Fa0/3      on          802.1q      trunking     99

Port   vlans allowed o trunk

Fa0/3  10,20,99

Port    Vlans allowed active in management domain

Fa0/3  10,20,99

Port    Vlans in spanning tree forwarding state and not pruned

Fa0/3  10,20, 99

The interface Fa0/3 on switch S3 is correctly configured to allow VLANs 10, 20, and 99. While the interface fa0/3 on switch S1 is incorrectly configured to allow only 10 and 99. This suggests that there is configuration error on S1.


The Fa0/1 and the Fa0/3 ports on S1 need to be reconfigured using the switchport trunk allowed vlan 10, 20, 99 command.


S1#config t

S1(config)#interface fa0/3

S1(config-if)#switchport trunk allowed vlan 10,20,99



Then use the show interfaces trunk command to very your configuration;

S1#show interface trunk

Port      Mode            Encapsulation           Status            Native vlan

Fa0/1   on                 802.1q                 trunking            99

Fa0/3   on                 802.1q                 trunking            99

Port      Vlans allowed on trunk

Fa0/1   10, 20, 99

Fa0/3   10, 20, 99

The output above shows that VLANs 10, 20, and 99 are now added to the Fa0/1 and Fa0/3 ports on switch S1.

Connectivity can also be verified by pinging from the PC5


Pinging with 32 bytes of data:

Reply from Reply from bytes-32 times-147ms TT-128

Reply from Reply from bytes-32 times-147ms TT-128

Reply from Reply from bytes-32 times-147ms TT-128

Incorrect VLAN IP Subnets Configurations

VLAN Trunking

Troubleshooting VTP configuration

VLAN Configuration

Native VLAN mismatches

Troubleshooting VLAN and Trunks

Managing VLAN  a VTP Enabled network

How to Configure a Trunk

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