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Difficulty: EasyInterswitch Connectivity and Trunking (802.1Q)

A network administrator is configuring an 802.1Q trunk link between two Cisco Catalyst switches. By default, how does an 802.1Q trunk interface handle Ethernet frames associated with the native VLAN when forwarding them across the trunk?

  1. It transmits the frames across the trunk link without inserting an 802.1Q VLAN header tag.Answer
  2. B
    It drops all native VLAN frames unless PortFast is explicitly enabled on the trunk interface.
  3. C
    It always inserts a VLAN tag with ID 1, regardless of which VLAN ID is configured as the native VLAN.
  4. D
    It uses Layer 2 discovery protocols (CDP/LLDP) TLVs to encapsulate and tag the native VLAN traffic.

Answer

By default, an 802.1Q trunk interface transmits frames belonging to the configured native VLAN without adding a 4-byte 802.1Q tag.
Under IEEE 802.1Q trunking specifications, frames belonging to the configured native VLAN are transmitted across the trunk link without an added 802.1Q VLAN header tag. When the receiving switch accepts untagged frames on an 802.1Q trunk port, it implicitly assigns those frames to its local native VLAN.

Step-by-Step Solution

1
Identify the frame tagging mechanism defined by IEEE 802.1Q for interswitch trunk links.
IEEE 802.1Q inserts a 4-byte VLAN tag field into Ethernet frames for all non-native VLAN traffic.
Trunk interfaces must distinguish traffic belonging to different VLANs over a shared physical medium.
2
Determine how 802.1Q treats traffic associated with the native VLAN.
Frames belonging to the native VLAN are forwarded untagged by default.
This behavior ensures backward compatibility with legacy devices and non-trunking switches that do not understand 802.1Q tags.

Key Concept

802.1Q Native VLAN Untagged Frame Forwarding
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Topics

Interswitch Connectivity and Trunking (802.1Q)

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