What is VLAN Trunk Modes Mismatch? Troubleshooting Example

By | 9th November 2015

Error#2.

How to Troubeshoot Trunk Mode Mismatches

A switch port Trunk link is configured manually with the switchport mode trunk command. Trunk ports use DTP advertisements to negotiate the state of the link with the remote switch port. When a port on a trunk link is configured with a trunk mode that is incompatible with the other trunk port, a trunk link fails to form between the two switches.

In this situation, the user computer PC5 (student) cannot connect to the internal web server. Again, using the topology diagram below shows a correct configuration. Let try and locate and solve the problem.




Trunk modes mismatches

The first thing you do here is to verify the status of the trunk ports on switch SW1 using the show interfaces trunk command.

SW1#show interfaces trunk

Port     Mode   Encapsulation     Status      Native vlan

Fa0/1  on         802.1q                   trunking    99

Port Vlans allowed on trunk

Fa0/1  10, 99

Port Vlans allowed and active in management domain

Fa0/1  10, 99

Port Vlans in spanning tree forwarding state and not pruned

Fa0/1 10,  99

Name:  Fa0/3

Switchport:  Enabled

Administrative Mode: dynamic auto

From the details above, it shows that interface Fa0/3 on switch SW1 is not configured as trunk port. On close examination you will discover that the Fa0/3 interface port is set to dynamic auto mode.




Lets look at the trunks on switch SW3 using the show interface trunk command:

SW3#show interfaces trunk

SW3#show interface fa0/3 switchport

Name: Fa0/3

Switchport: Enabled

Administrative Mode:  dynamic auto

The above detail also reveals that there are no active trunk ports. Further check reveals that the Fa0/3 interface is also in dynamic auto mode; this explains why the trunk is down.

Solution:

To activate the trunk link, you need to reconfigure the trunk mode of the Fast Ethernet Fa0/3 ports on switches SW1 and SW3.

Configuration example on SW1:

SW1#config t

SW1(config)#interface fa0/3

SW1(config-if)#switchport mode trunk

SW1(config-if)#end

And use the show interfaces fa0/3 switchport to verify configuration on SW1:

SW1#show interfaces fa0/3 switchport

Name: Fa0/3

Switchport: Enabled

Administrative Mode: trunk

Reconfigure switchport fa0/3 as trunk link on Switch 3

SW3#config t

SW3(config)#interface fa0/3

SW3(config-if)#switchport mode trunk

SW3(config-if)end

Use the show interfaces fa0/3 switchport to verify configuration on Sw3

SW3#show interfaces fa0/3 switchport

Name:  Fa0/3

Switchport:  Enabled

Administrative Mode:  trunk

…..

Use the show interfaces trunk on Switch 3 to verify connectivity

SW3#show interfaces trunk

Port     Mode   Encapsulation   Status      Native vlan

Fa0/3  On        802.1q                 trunking   99

Port Vlans allowed on trunk

Fa0/1  10, 99

Port Vlans allowed and active in management domain

Fa0/3  10, 99

Port Vlans in spanning tree forwarding state and not pruned

Fa0/3 10,  99

S3#

The above detail shows that interface Fa0/3 has been reconfigured as a trunk.

Now ping from the PC5 command line to very connectivity:

PC5> 192.168.10.30

Pinging  192.168.10.30 with 32 bytes of data

Reply from 192.168.10.30: Reply from 192.168.10.30: bytes-32 times-147ms TT-128

Reply from 192.168.10.30: Reply from 192.168.10.30: bytes-32 times-147ms TT-128

Reply from 192.168.10.30: Reply from 192.168.10.30: bytes-32 times-147ms TT-128

The output from PC5 above indicates that connectivity has been regained to the WEB/TFTP server found at IP address 192.168.10.30.




Error#3 – Incorrect VLAN configuration

Other Methods of Troubleshooting VTP

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