What actually breaks when native VLANs are mismatched on a trunk?
CDP is complaining about a native VLAN mismatch on our uplink, but user traffic mostly seems fine. What is really at risk here?
1 Answer
The native VLAN is the one that crosses a trunk untagged. If side A says native 10 and side B says native 20, an untagged frame that leaves A as "VLAN 10 traffic" is received by B and dropped into VLAN 20 — traffic silently jumps VLANs, no error, no drop counter. "Mostly fine" is exactly the danger: tagged VLANs work normally, so the leak only affects untagged traffic (management protocols, anything in the native VLAN), which is why it lurks until something weird happens — hosts receiving foreign broadcasts, intermittent management reachability, or a VLAN-hopping security hole. Fix: make native match on both ends (switchport trunk native vlan X), and best practice is a dedicated unused VLAN as native rather than VLAN 1.