Lab Objective:
Learn how to troubleshoot a VLAN mismatch.
Lab Purpose:
Troubleshooting VLAN mismatches features in the exam syllabus. You will really only see these on trunk links, which must have the same native VLAN configured on either side. The only other VLAN mismatch you might encounter is hosts in the same VLAN being in different subnets, which isn’t permitted.
Lab Tool:
Packet Tracer
Lab Topology:
Please use the following topology to complete this lab exercise:
Lab Walkthrough:
Task 1:
Drag two switches onto the canvas and connect them with a crossover cable. I used interface G0/1 in this lab, but you can use any interface.
Task 2:
Set the interface to trunk and set the native VLAN to 10.
Switch#config t
Switch(config)#host SW0
SW0(config)#int g0/1
SW0(config-if)#switchport mode trunk
SW0(config-if)#switchport trunk native vlan 10
SW0(config-if)#end
Task 3:
Cisco Discovery Protocol (CDP) will begin to generate error messages, which will be printed on your console session.
%CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on GigabitEthernet0/1 (10), with Switch GigabitEthernet0/1 (1).
Task 4:
Two commands will tell you what the native VLAN on the trunk is. Issue both of the commands below.
SW0#show interfaces trunk
Port Mode Encapsulation Status Native vlan
Gig0/1 on 802.1q trunking 10
Port Vlans allowed on trunk
Gig0/1 1-1005
Port Vlans allowed and active in management domain
Gig0/1 1
Port Vlans in spanning tree forwarding state and not pruned
Gig0/1 1
SW0#show interfaces g0/1 switchport
Name: Gig0/1
Switchport: Enabled
Administrative Mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: dot1q
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 10 (Inactive)
Voice VLAN: none
Task 5:
As the administrator, you would know what the correct native VLAN should be. You would then configure SW1 G0/1 to use the same native VLAN (after checking the configuration by issuing one of the previous two commands first).
Switch>en
Switch#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)#host SW1
%CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on GigabitEthernet0/1 (1), with SW0 GigabitEthernet0/1 (10).
SW1(config)#int g0/1
SW1(config-if)#switchport trunk native vlan 10
SW1(config-if)#end
Note:
As mentioned earlier in this guide, you would usually set your native VLAN to a high number and ensure it was unused by hosts.
