What is VLAN Configuration Errors and Solution

By | 9th November 2015

Error#1 Native VLAN Mismatches

Lets assume that one of the workstations on the network (PC 5) cannot connect to the internal web server WEB/TFTP, lets use the figure below as an example of a switched network, the first place you start will be at the Switch 2 to check if VLANs are properly configured. Looking at the diagram, switch port fa0/3 on Switch 2 is configured as a trunk port.

                                                                             Native VLAN Mismatch 

Native VLAN mismatches

When you connect to switch S2, if there is an error on the switch port, it will appear on your console window, in this case there is and it looks like this:


#CDP-4-NATIVE VLAN_MISMATCH: Native VLAN mismatch discovered on

FastEthernet0/3  (100), with S1  FastEthernet0/3  (99).

Using the show interfaces fa0/3 switchport command will display connectivity detail on the port.


S2# show interfaces fa0/3 switchport

Name:     fa0/3

Switchport:  Enabled

Administrative Mode:  trunk

Operational Mode:  trunk

Administrative Trunking Encapsulation:  dot1q

Operational Trunking Encapsulation:  dot1q

Nagotiation of Trunking:  On

Access Mode VLAN:  1 (default)

Trunking Native Mode VLAN: 100 (Inactive)

Trunking VLANs Enabled:  10,  99

Looking at the above detail, you will notice that the native VLAN has been set to VLAN 100 and it is inactive.

As you look further down the output, you see that the allowed VLANs are 10 and 99.

To sum it up, this is a case of a mismatched native VLAN

The solution:

You need to reconfigure the native VLAN on the Fast Ethernet F0/3 trunk port to be VLAN 99.  e.g

S2#config t

S2#interface fa0/3

S2#switchport trunk native vlan 99

After you have done that, use the show interfaces fa0/3 switchport confirm you configuration.

And use the Ping to confirm connectivity with the server.  e.g.

PC5> ping

Pinging with 32 bytes of data:

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

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

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

The screen output for the computer PC5 shows that connectivity has been restored to the WEB/TFTP server found at IP address

Error#2 Trunk Mode Mismatches

Other Trunk errors and solutions

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