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Difficulty: EasyInterswitch Connectivity and Trunking (802.1Q)

Which TWO statements accurately describe standard IEEE 802.1Q trunking behavior on Ethernet switch interfaces? (Select TWO.)

  1. By default, traffic belonging to the native VLAN is transmitted across the trunk link without an added 802.1Q VLAN tag.Answer
  2. B
    Native VLAN IDs can differ between opposing ends of an active trunk link without impacting traffic forwarding or causing errors.
  3. An 802.1Q tag header inserts a 4-byte field containing a VLAN Identifier (VID) into the Ethernet frame header for non-native VLAN traffic.Answer
  4. D
    Spanning Tree Protocol PortFast should be enabled on interswitch trunk interfaces to minimize link startup convergence delay.

Answer

The correct statements are that native VLAN frames are transmitted untagged by default across an 802.1Q trunk link, and that 802.1Q encapsulation inserts a 4-byte header containing the VLAN ID into non-native VLAN frames.
Under IEEE 802.1Q trunking, traffic belonging to the native VLAN is sent untagged by default without an added header. For all other VLANs traversing the trunk, the switch inserts a 4-byte tag header into the original Ethernet frame containing the VLAN ID.

Step-by-Step Solution

1
Analyze frame tagging behavior for 802.1Q trunking
Recognize that 802.1Q inserts a 4-byte VLAN tag into Ethernet frames for tagged VLANs, but leaves native VLAN frames untagged by default.
This dual behavior allows legacy untagged devices and native traffic to traverse trunk links.
2
Evaluate native VLAN configuration requirements across trunk links
Confirm that native VLAN IDs must match on both ends of the interswitch trunk.
Mismatched native VLAN IDs cause frames originating in one VLAN to exit into a different VLAN on the receiving switch.
3
Evaluate Spanning Tree Protocol (STP) PortFast application
Confirm that PortFast is intended strictly for edge access ports connected to end-user devices.
Enabling PortFast on switch-to-switch interconnect trunks can lead to catastrophic bridging loops during topology changes.

Key Concept

IEEE 802.1Q Trunk Header Mechanics and Native VLAN Operation
Estimated Time:1m 0s
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Topics

Interswitch Connectivity and Trunking (802.1Q)

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