Why is the trunk link tagged on the frames?
Doesn't every frame have a VLAN identifier? So why is it labeled on the trunk link to identify the VLAN?
For example, in this image, computer A is in red VLAN and the switch detects this, so why re-tagging in the trunk link?
Solution 1:
No – frames sent by regular devices on access ports have no VLAN tag whatsoever. The switch only "detects" their VLAN based on static configuration stored in the switch itself (i.e. each port's PVID parameter), not based on frame contents.