The best approach to mitigating double-tagging attacks is to ensure that the native VLAN of the trunk ports is different from the VLAN of any user ports.
Double-tagging attack takes advantage of the way that hardware on most switches de-encapsulate 802.1Q tags.
Most switches perform only one level of 802.1Q de-encapsulation, allowing an attacker to embed a second, unauthorized attack header in the frame.
After removing the first and legit 802.1Q header, the switch forwards the frame to the VLAN specified in the unauthorized 802.1Q header.
The best approach to mitigating double-tagging attacks is to ensure that the native VLAN of the trunk ports is different from the VLAN of any user ports.