1) each port on a switch is a collision domain this is called microsegmentation and you can run full-duplex so the answer is No there are no more collisions in a switched network. apart from shared segment where there is a hub or AP attached
2)on links where multiple VLANS can travel you must have a way of differentiating the VLAN the frame is part of and so you have a field with the VLAN id which is a Tag permitting to recognize the VLAN so the switch knows on which port it can forward the frame when the tag is removed.So a frame enters a port belonging to a particuliar vlan and it is tagged with the vlan id when traversing the trunk( or tagged port) and untagged when forwarded to the destination.
In 802.1Q ther is also the concept of native vlan : frames from this vlan will have no tag, this is a compatibility feature for devices not VLAN aware.
Alain's explanation basically covers all you require but just to add a few lines:
first question: each port in a switch is a seperate collision domain, so there are no collisions usually in a switch except if probably(ususally rare) and error in config ocuurs.
second question: the vlan tag just enables the switch to know what packet being transmitted belongs to what vlan. it is usually shared between the switches and dropped before the packet is transmitted to the PC it is meant for. But Cisco IP phones also allow tagging to take place.
1. Collision takes place only on Media and not inside any device . So there is no collision possible in Ethernet Switch.
2. There are two mechanisms of Frame tagging a. ISL b. 802.1q out of which ISL is no more used . The VLAN Identifier is attached to normal Ethernet frame is called as Frame Tagging . For ISL it would have been 26 + 1518 + 4 , here 26 Bytes is header 4 bytes trailer 1518 is considered as Ethernet Frame . This adding of 26+4 is called as Tagging .
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