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Difficulty: EasyInterswitch Connectivity and Trunking (802.1Q)

In an IEEE 802.1Q trunk link between two Cisco switches, how is traffic belonging to the native VLAN transmitted across the link by default?

  1. It is transmitted as untagged frames without an 802.1Q header.Answer
  2. B
    It is tagged with a VLAN ID of 0 to indicate priority-only traffic.
  3. C
    It is dropped until PortFast is explicitly enabled on the switch interface.
  4. D
    It is encapsulated within CDP messages to exchange VLAN metadata.

Answer

Traffic belonging to the native VLAN is transmitted as untagged frames without an 802.1Q header by default.
By default, IEEE 802.1Q trunking transmits frames belonging to the native VLAN across the link without inserting an 802.1Q VLAN header tag. When the receiving switch accepts untagged frames on a trunk port, it automatically places them into its configured native VLAN.

Step-by-Step Solution

1
Identify the standard frame handling behavior of IEEE 802.1Q trunking.
IEEE 802.1Q inserts a 4-byte tag field into Ethernet frames for standard tagged VLANs to preserve VLAN identification across switch interconnects.
Tagging allows switches to multiplex multiple VLANs over a single physical link.
2
Apply the native VLAN exception rule.
Frames associated with the native VLAN are sent across the trunk link completely untagged without modifying the original frame header.
This behavior maintains backward compatibility with legacy devices or unmanaged switches that do not process 802.1Q tags.

Key Concept

802.1Q Native VLAN Untagged Behavior
Estimated Time:45s
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Topics

Interswitch Connectivity and Trunking (802.1Q)

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