Question

Difficulty: MediumVLAN Configuration and 802.1Q Trunking

Match each 802.1Q trunking component or port mode behavior on the left with its corresponding operational description on the right.

  • Native VLAN egress behaviorForwards frames across an 802.1Q trunk link without inserting a 4-byte VLAN tag header.
  • Tagged VLAN egress behaviorInserts a 4-byte header containing a 12-bit VLAN ID into the Ethernet frame before transmitting over a trunk link.
  • Standard access port ingress behaviorAccepts untagged Ethernet frames, assigns them to its configured access VLAN, and drops incoming tagged frames.
  • 802.1Q Tag Control Information (TCI)Contains 3 bits for Priority Code Point (PCP/CoS), 1 bit for Drop Eligible Indicator (DEI), and 12 bits for VLAN ID (VID).

Answer

Native VLAN egress behavior matches forwarding frames untagged across the trunk. Tagged VLAN egress behavior matches inserting a 4-byte header containing a 12-bit VLAN ID. Standard access port ingress behavior matches accepting untagged frames, assigning them to the configured access VLAN, and dropping tagged frames. 802.1Q Tag Control Information (TCI) matches containing PCP/CoS priority bits, DEI, and the 12-bit VLAN ID.
The correct pairings accurately reflect IEEE 802.1Q header structures and switchport operations: Native VLAN egress traffic flows untagged; non-native VLAN traffic has a 4-byte header with a 12-bit VLAN ID inserted; access ports accept untagged host frames for a single broadcast domain while discarding tagged frames; and the 802.1Q TCI field encodes 3-bit CoS priority, 1-bit DEI, and the 12-bit VLAN identifier.

Step-by-Step Solution

1
Analyze Native VLAN trunking behavior
Native VLAN traffic travels across 802.1Q trunk links untagged without modifying the original frame encapsulation.
IEEE 802.1Q trunking specifies that the native VLAN carries untagged frames across interconnecting switches for backward compatibility.
2
Analyze Tagged VLAN trunking behavior
Non-native VLAN frames receive a 4-byte 802.1Q tag containing a 12-bit VLAN Identifier (VID).
Explicit tagging enables switches on both ends of the trunk to correctly segregate traffic into distinct logical networks.
3
Analyze Access Port ingress behavior
Access ports receive untagged traffic from host interfaces and assign it to a single target VLAN.
Access ports belong strictly to one VLAN domain and discard incoming tagged frames for security and operational integrity.
4
Analyze 802.1Q TCI subfields
The TCI subfield allocates 3 bits for Class of Service (CoS), 1 bit for DEI, and 12 bits for VID.
The 12-bit VID allows support for up to 4,094 usable VLANs (21222^{12} - 2).

Key Concept

VLAN Configuration and 802.1Q Trunking Mechanics
Rate this question