With 802.1Q trunking, frames from the native VLAN are not tagged. Frames from all other VLANs are tagged.
If the native VLAN on one end of the trunk is different than the native VLAN on the other end, the traffic of the native VLANs on both sides cannot be transmitted correctly on the trunk.
The native VLAN is VLAN 1 by default, but may be configured.

On each switch, use the following commands to complete this lab:
SwitchA>ena
SwitchA#conf t
SwitchA(config)#int range gi 0/1 - 2 SwitchA(config-if-range)#switchport trunk native vlan 10
(press Ctrl + Z)
SwitchA#copy run start