Which VLAN tagging method is most commonly used on modern Cisco switches?
Correct Answer: B
Detailed Explanation: VLAN trunking is the technique used to carry multiple VLANs over a single physical link between switches. To maintain the segregation of traffic for each VLAN, frames must be tagged with an identifier indicating the VLAN to which they belong. There are two main encapsulation methods historically used for trunking on Cisco switches: Cisco’s proprietary Inter-Switch Link (ISL) and the IEEE standard 802.1Q. ISL (Inter-Switch Link): ISL is a Cisco proprietary encapsulation protocol. It encapsulates Ethernet frames in a new header and trailer, which includes the VLAN identification information. However, ISL has largely fallen out of favor due to its proprietary nature and the fact that it adds additional overhead to each frame. IEEE 802.1Q: IEEE 802.1Q is an industry-standard VLAN tagging protocol that inserts a 4-byte tag into the original Ethernet frame. This tag contains a 12-bit VLAN identifier (VID), allowing for up to 4096 VLANs. Because 802.1Q is an open standard, it is supported by equipment from multiple vendors, making it the de facto choice for modern network designs. In addition to its wide acceptance, 802.1Q introduces less overhead than ISL, and it allows for the coexistence of tagged and untagged (native) traffic on the same trunk port. Comparison and Industry Adoption: Modern Cisco switches primarily support IEEE 802.1Q as the trunking encapsulation method, especially in mixed-vendor networks. While some legacy environments might still use ISL, most networks have transitioned to 802.1Q due to its interoperability, efficiency, and simplicity. Other Options: Frame Relay (Option C) is a WAN technology and is not used for VLAN tagging on LAN segments. VLAN Trunking Protocol (VTP) (Option D) is used for managing VLAN configurations across switches but is not a trunk encapsulation method. Practical Implications: When configuring trunk ports on Cisco switches, the command used to set the encapsulation is typically: switchport trunk encapsulation dot1q This explicitly sets the trunking method to IEEE 802.1Q. In many modern switches, 802.1Q is the only supported encapsulation, so the command may not even be required if the switch defaults to 802.1Q. Conclusion: IEEE 802.1Q (Option B) is the most commonly used VLAN tagging method on modern Cisco switches. Its support as an open standard, reduced overhead, and broad vendor interoperability make it the preferred choice for trunking in today’s networks.
This CCNA practice question helps students prepare for Cisco networking certification exams by testing knowledge of network fundamentals, routing, switching, and network security concepts.