Native VLAN between trunk ports detail

[net guru]

Hello,

I have a question that I can't just find a clear response to it about the MUST of configuring the same native vlan between two switch ports. in the CCNP ENCOR study guide , I've read this " The native VLAN should match on both trunk ports, or traffic
can change VLANs unintentionally" .

I don't get what is meant by "traffic change VLANs unintentionally" , can anyone give me a scenario explaining the behaviour in such case ?

Thanks alot !

[balaji.bandi · Accepted solution]

The Native VLAN is simply the one VLAN that traverses a Trunk port without a VLAN tag.

Native VLAN has to match on both sides of the trunk - why?

Because they tag native VLAN traffic, take the example you are sending information X VLAN other Y VLAN, so they mismatch, they can not communicate the information as expected.

you can clearly see the logs native VLAN mismatch when you configure different VLAN both the side.

if not mentioned any native VLAN, VLAN 1 is the default.

[net guru]

ah okey , its clear now ! traffic from a SWITCH-A view belonging to VLAN 2 sent to SWITCH-B who has VLAN-2 as native VLAN will be untagged which is wrong if SWITCH-A has VLAN 1 as native vlan .

makes sense !

thanks bandi!