Native VLAN trunking across a network

[stuartduperron]

Hello Cisco community,

I have a pretty specific question and can't find a good example of it anywhere.

In our network we have hypervisors sending untagged traffic across a trunk port to two switches, lets call them switch 1 and 2. On switch 1 and 2 the native vlan for the interfaces pointing to the hypervisors are set to 50. The trunk port interconnect between the two switches has its native vlan set to 1100 with vlan 50 being one of the allowed vlans. Would this mismatch of native vlans affect traffic? To clarify the native vlan on both sides of all trunks are the same.

I was originally thinking that since the traffic coming up to the switches from the hypervisor was untagged that the interconnect would not allow it since the allowed vlan 50 was checking for vlan 50 tags.

Example config for the interconnect below:

interface **insert name**

switchport mode trunk

switchport trunk native vlan 1100

switchport trunk allowed vlan 50,60,70

speed 1000

duplex full

example for hypervisor config:

interface **insert name**

switchport mode trunk

switchport trunk native vlan 50

switchport trunk allowed vlan 50,60,70

speed 1000

duplex full

Hopefully this question makes sense, thank you in advance.

[M02@rt37]

Hello @stuartduperron,

In your scenario, where you have hypervisors sending untagged traffic across a trunk port to two switches (switch 1 and switch 2), the native VLAN mismatch should not directly affect the traffic. However....the native VLAN is used for untagged traffic. As long as the native VLAN on both sides of the trunk (between switch 1 and switch 2) is the same, the untagged traffic should be properly transmitted between the switches. So, in your configuration, where both sides have the native VLAN set to 1100, there is no native VLAN mismatch between the switches.

Also, the native VLAN for the hypervisor interfaces (set to VLAN 50) is relevant for traffic originating from the hypervisors. Since you mentioned that the traffic from the hypervisors is untagged, it will be placed into the native VLAN (VLAN 50) by switch 1 and switch 2.

On the trunk interconnect between the two switches, you have allowed VLANs 50, 60, and 70 ---> means that these VLANs are allowed to traverse this interconnect. As long as VLAN 50 is allowed, untagged traffic from the hypervisors (which belongs to VLAN 50) should pass through this trunk without issues.

[Joseph W. Doherty]

"Would this mismatch of native vlans affect traffic?"

It would not because there's no mismatch.

I suspect you slightly misunderstand what a "native" VLAN is, which is just a per trunk port configuration setting that denotes which VLAN's frames are to be transmitted without a VLAN tag and to which VLAN frames are to be considered if received without a VLAN tag.  A "native" VLAN has no meaning through a switch.