How to configure native VLAN and fixing the common native VLAN mismatch issue
Native VLAN – Native VLAN allows untagged traffic to be part of one particular VLAN. Native VLAN functionality is used when untagged traffic is received on the trunk connection of the switch.
In the above image, you can see that the hub is connected to the switch. VLAN and tagging are switching technology and as Hub is unable to tag the traffic then what will happen if PC 4 and PC 5 which are connected to the Hub send the traffic? In that case, we have to specify native VLAN on both of the switch interfaces that are connected to Hub and that will help when the switch will receive the untagged traffic on those interfaces then it will be the part of the specified VLAN which we have configured as Native VLAN.
Although Hubs are not in use now a day’s however in a small organization where the budget is low and organizations cannot afford to spend money on new network equipment and they want to use the old network devices then the network admin may have to work with such topologies.
In small networks, speed is not the priority for the company so using the device as a hub does not harm the network.
Another network scenario, in which native VLAN is used commonly, is shown below. IP phones are used commonly in companies these days, they allow daisy-chaining of end devices like PC which saves the interfaces and saves the extra cost of the telecom.
Daisy-chaining the PC is an efficient solution as an organization does not have to buy extra switches to implement IP phones.
PC 6 cannot tag the traffic so we have to configure Native VLAN to route the traffic generated by the pc on the desired VLAN however, the Cisco IP phone can tag the traffic so native VLAN will only be used by the PC.
By default, native VLAN is configured as 1 on all the interfaces however, in case we want to route traffic to another VLAN then we have to change the native VLAN accordingly.
To check the native VLAN that is configured, we can use the command ‘show interfaces trunk’
To configure the native VLAN or to change the default native VLAN, we have to use the following command.
Switch(config)#interface fastEthernet 0/24
Switch(config-if)#switchport trunk native vlan 5
We have to enter into interface configuration mode and then use the command shown above; we have to specify the VLAN number where we want to redirect untagged traffic. In the above example, traffic will be routed to VLAN 5.
Native VLAN mismatch
Native VLAN mismatch happens when interfaces of the switch are configured with different native VLANs and in that case traffic generated by the PC will be routed to both VLANs which will cause issues in the network.
The good thing about the switch is that it detects the native VLAN itself and the following message is displayed on the Command-line interface of the device.
— %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on FastEthernet0/24 (5), with Switch FastEthernet0/24 (1) —
To fix the Native VLAN mismatch, we have to configure the same native VLAN on all interfaces that are connected to the hub so the traffic generated will only be redirected to one VLAN.
In this lab, we have intentionally created a native VLAN mismatch so you can download this lab and can practice fixing the mismatch by assigning the same Native VLAN on both switches.
You will be able to see the mismatch message on the CLIs of both of the switches.