In a VLAN enabled switched network, what is meant by the term frame tagging when traffic is sent over a trunk link between switches or between a switch and a router?

Introduction / Context:
In VLAN aware networks, switches and routers often need to carry traffic for multiple VLANs across a single physical link. To keep VLANs logically separate while sharing this link, devices must indicate in each frame to which VLAN the frame belongs. This process is known as frame tagging. Understanding the concept of frame tagging is essential for working with 802.1Q and other trunking technologies in Cisco based networks.

Given Data / Assumptions:

• Multiple VLANs are configured in the network.
• Switches or routers are connected with trunk links that carry traffic for more than one VLAN.
• The term frame tagging is used in the context of these trunk links.
• The question focuses on what information is inserted into the frame and why.

Concept / Approach:
Frame tagging refers to the method of inserting VLAN identification into Ethernet frames when they traverse a trunk link. Technologies such as IEEE 802.1Q insert a small header field in the Ethernet frame that contains the VLAN ID and some control information. Cisco ISL uses encapsulation to include VLAN information. When a tagged frame arrives at the other end of the trunk, the receiving device reads the VLAN information and forwards the frame only to ports that belong to that VLAN. This allows many VLANs to be multiplexed over one physical connection while preserving logical separation.

Step-by-Step Solution:
1. Identify that the question is specifically about VLANs and trunk links. 2. Recall that trunk ports must be able to distinguish which VLAN each frame belongs to as they carry traffic for multiple VLANs simultaneously. 3. Understand that frame tagging is the process of adding VLAN related information into each frame. 4. Recognize that IEEE 802.1Q does this by inserting a VLAN tag into the Ethernet frame header, while other methods like ISL encapsulate the frame. 5. Conclude that frame tagging is about inserting VLAN identification information so that multiple VLANs can share a trunk link.

Verification / Alternative check:
To verify, you can look at packet captures on trunk links. Tools such as Wireshark display a VLAN tag field when capturing frames on an 802.1Q trunk. This field shows the VLAN ID and confirms that each frame carries explicit VLAN information. Access ports, by contrast, do not use tagging and send frames without VLAN headers to end hosts. This difference between tagged trunk traffic and untagged access traffic demonstrates that frame tagging is specifically a trunk link concept for multiplexing VLANs.

Why Other Options Are Wrong:
Option a: Adding a sequence number to frames is related to higher layer protocols or specialized mechanisms, not typical VLAN frame tagging. Option b: Priority bits for Quality of Service may be present in the same 802.1Q header, but frame tagging in this context is primarily about VLAN identification, not just QoS marking. Option d: Encryption is a security feature handled by other technologies and is not the definition of frame tagging in VLAN scenarios. Option e: Checksums already exist in Ethernet frames as the Frame Check Sequence, but that is for error detection and not specific to VLAN frame tagging.

Common Pitfalls:
A common error is to confuse VLAN tagging with general header fields such as checksums or sequence numbers. Another pitfall is thinking that tagging happens on all ports, when in fact only trunk ports tag frames for VLAN identification. Access ports send and receive untagged frames for a single VLAN. Also, some learners mix up QoS marking with VLAN tagging because both use bits in the header, but they serve different purposes. Always remember that the central idea of frame tagging in this context is to carry VLAN identification across shared links.

Final Answer:
Frame tagging is inserting VLAN identification information into Ethernet frames so that multiple VLANs can share a single trunk link while remaining logically separate.