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Difficulty: MediumInterswitch Connectivity and Trunking (802.1Q)

A network administrator is examining the operational mechanics of an IEEE 802.1Q trunk link connecting two Cisco switches. Which two statements accurately describe the frame tagging and forwarding behavior of IEEE 802.1Q trunking? (Select two.)

  1. Frames belonging to the native VLAN are forwarded untagged across the trunk link by default.Answer
  2. A 4-byte tag header containing the VLAN ID is inserted into the Ethernet frame header for non-native VLAN traffic.Answer
  3. C
    Configuring PortFast on the trunk interface accelerates dynamic trunk negotiation while preventing Spanning Tree Protocol loops.
  4. D
    If the native VLAN IDs are mismatched across the link, untagged traffic is automatically discarded by the receiving switch interface.

Answer

IEEE 802.1Q forwards native VLAN frames untagged by default and inserts a 4-byte VLAN tag header into Ethernet frames for all non-native VLAN traffic.
IEEE 802.1Q trunking inserts a 4-byte tag header (including the Tag Protocol Identifier and Tag Control Information) between the Source MAC address and EtherType fields of Ethernet frames for non-native VLAN traffic. For native VLAN traffic, frames are forwarded untagged across the link by default to maintain compatibility.

Step-by-Step Solution

1
Analyze standard 802.1Q frame tagging mechanics.
Identify that 802.1Q inserts a 4-byte header into tagged frames and sends native VLAN frames untagged by default.
802.1Q maintains backwards compatibility with untagged legacy device traffic via the native VLAN concept while identifying other VLANs using the 4-byte tag field.
2
Evaluate the effects of native VLAN mismatches and PortFast configuration on trunk links.
Confirm that native VLAN mismatches lead to cross-VLAN traffic leakage rather than dropping frames, and PortFast on trunk links creates bridging loops.
PortFast bypasses Spanning Tree listening/learning phases, and untagged traffic is processed as belonging to the local native VLAN upon receipt regardless of the sender's VLAN ID.

Key Concept

IEEE 802.1Q Trunking Frame Tagging and Native VLAN Operations
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Topics

Interswitch Connectivity and Trunking (802.1Q)

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