Which Cisco VLAN tagging method uses an external encapsulation header around the original Ethernet frame so that multiple VLANs can be multiplexed over a trunk link, rather than inserting a tag inside the existing Ethernet header as IEEE 802.1Q does?

Introduction / Context:
Cisco networks historically supported two major methods for carrying VLAN information across trunk links: IEEE 802.1Q and the Cisco proprietary Inter Switch Link (ISL). Both approaches allow multiple VLANs to share a single physical link, but they handle tagging differently. IEEE 802.1Q inserts a VLAN tag inside the existing Ethernet header, while ISL encapsulates the original Ethernet frame inside a new header. This question asks you to identify which Cisco method uses external encapsulation for VLAN tagging.

Given Data / Assumptions:

• Multiple VLANs must be transported over a trunk link between Cisco devices.
• Some methods insert tags within the frame header, others use encapsulation around the frame.
• The focus is on the Cisco proprietary method that encapsulates the original frame.
• Technologies unrelated to VLAN trunking, such as MPLS, are also listed as distractors.

Concept / Approach:
ISL is a Cisco proprietary VLAN trunking protocol that wraps the entire original Ethernet frame with a new ISL header and trailer. This is called external encapsulation because the VLAN information is added outside the existing frame. IEEE 802.1Q, by contrast, modifies the Ethernet header by inserting a VLAN tag field inside it. MPLS works at a different layer and uses labels to forward packets through label switched paths, not for standard layer 2 VLAN trunking. VTP is a management protocol for distributing VLAN information between switches, not a trunk encapsulation method. Understanding these distinctions allows you to correctly identify ISL as the external encapsulation method.

Step-by-Step Solution:
1. Recall that Cisco supported both ISL and IEEE 802.1Q for VLAN trunking on some platforms. 2. Remember that IEEE 802.1Q inserts a tag inside the Ethernet frame header, modifying the frame but not fully encapsulating it. 3. Recognize that ISL takes the whole original frame and encapsulates it with an additional header and trailer that carry VLAN ID and other information. 4. Consider the wording external encapsulation method in the question, which corresponds directly to the ISL approach. 5. Eliminate options that refer to technologies not used for basic VLAN trunk encapsulation on Ethernet links.

Verification / Alternative check:
Cisco documentation describes ISL as encapsulating frames with a 26 byte header and a 4 byte trailer, which include VLAN ID and integrity information. Packet captures on old platforms that still support ISL show the original Ethernet frame fully intact inside an ISL header. In contrast, 802.1Q adds a 4 byte tag between the source MAC address and the EtherType field in the existing header. This confirms that ISL uses external encapsulation, matching the description in the question.

Why Other Options Are Wrong:
Option a: IEEE 802.1Q tagging inserts a VLAN tag inside the Ethernet frame header; it does not encapsulate the frame externally. Option c: MPLS label switching uses labels to forward packets in a service provider environment and operates differently from simple layer 2 VLAN trunking on Ethernet links. Option d: Token Ring source route bridging is an older technology that is not used for Ethernet VLAN trunk encapsulation. Option e: VTP operates as a control protocol to distribute VLAN information between switches, but it does not define how frames are tagged or encapsulated on trunks.

Common Pitfalls:
Learners sometimes assume that any VLAN mechanism involving Cisco must refer to 802.1Q, since ISL is less common on modern equipment. Another frequent confusion is between VTP and trunking methods. VTP manages VLAN definitions but does not carry user data frames itself. On exams, paying attention to keywords such as external encapsulation versus header insertion will help you distinguish ISL from 802.1Q. Even though ISL is legacy on many platforms, understanding its characteristics remains useful for historical and exam oriented knowledge.

Final Answer:
The Cisco tagging method that uses external encapsulation to multiplex VLANs over a trunk is ISL (Inter Switch Link) encapsulation.