Native VLAN with 802.1Q
If you are using 802.1Q, specify a native VLAN for the trunk link with the
command:
(config-if)#switchport trunk native vlan vlan-num
Frames from the native VLAN are sent over the trunk link untagged. Native
VLAN is the VLAN the port would be in if it were not a trunk, and it must
match on both sides of the trunk link. VLAN 1 is the default native VLAN
for all ports.