Two switches are joined by an 802.1Q trunk. One switch is configured with native VLAN 1 on the trunk, the other with native VLAN 99. Spanning tree and CDP are running. What is the consequence of this configuration?
- AThe trunk forms normally and all VLANs pass, because the native VLAN only affects which VLAN carries management traffic and never the forwarding of user data.
- BThe trunk is administratively shut down by the switch as soon as the mismatch is detected through CDP, requiring a manual no shutdown to recover.
- CTraffic from the two native VLANs is merged so that frames in VLAN 1 on one side arrive in VLAN 99 on the other, creating a security and reachability problem. Correct
- DOnly VLAN 99 is permitted across the link, because the higher native VLAN number always takes precedence and prunes the lower-numbered native VLAN.
Why A is wrong: It is tempting because a native-VLAN mismatch does not bring the physical link down, but the mismatch does affect user data: untagged frames leak between the two different native VLANs.
Why B is wrong: CDP does log a native VLAN mismatch notice, but Cisco switches do not err-disable or shut the trunk for this; the port stays up and continues forwarding, which is what makes the problem subtle.
Why C is correct: Each switch sends its native-VLAN traffic untagged, and the neighbour places received untagged frames into ITS own native VLAN, so VLAN 1 and VLAN 99 traffic is bridged together across the link.
Why D is wrong: There is no rule that a higher native VLAN number wins or prunes another VLAN; native VLAN selection is purely about which VLAN is sent untagged, so this invented precedence is wrong.