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Difficulty: MediumVLAN Configuration and 802.1Q Trunking

A network engineer is configuring an 802.1Q trunk link between `SW-CORE-01` and `SW-ACCESS-05`. Interface `GigabitEthernet 0/1` on `SW-CORE-01` is configured with `switchport trunk native vlan 50`. Interface `GigabitEthernet 0/1` on `SW-ACCESS-05` retains the default configuration of `switchport trunk native vlan 1`. Both switches have VLANs 10, 20, and 50 allowed on the trunk link.

When an untagged frame originating from an access port assigned to VLAN 50 on `SW-CORE-01` is forwarded across the trunk link, how will `SW-ACCESS-05` handle this frame upon arrival?

  1. It will accept the untagged frame and forward it within VLAN 1 because VLAN 1 is configured as its local native VLAN.Answer
  2. B
    It will drop the frame immediately because the frame's VLAN ID does not match the local native VLAN configuration.
  3. C
    It will inspect the 802.1Q header tag, strip it, and place the frame directly into VLAN 50.
  4. D
    It will automatically perform Layer 2 routing to forward the frame from VLAN 50 to VLAN 1 without requiring a router.

Answer

SW-ACCESS-05 will accept the untagged frame and process it as part of VLAN 1 because untagged frames arriving on an 802.1Q trunk are automatically assigned to the receiving interface's configured native VLAN.
The correct response highlights that untagged frames arriving on an 802.1Q trunk interface are mapped directly to the local interface's native VLAN. SW-CORE-01 transmits VLAN 50 frames untagged because VLAN 50 is its native VLAN. SW-ACCESS-05 receives the untagged frame and associates it with VLAN 1, which is its own native VLAN setting.

Step-by-Step Solution

1
Analyze frame egress behavior at SW-CORE-01
Since VLAN 50 is configured as the native VLAN on SW-CORE-01's trunk port, traffic originating from VLAN 50 is transmitted across the 802.1Q trunk link without an 802.1Q VLAN tag.
By standard 802.1Q behavior, frames belonging to the native VLAN are sent untagged across trunk links.
2
Analyze frame ingress behavior at SW-ACCESS-05
SW-ACCESS-05 receives an untagged frame on its trunk port where `native vlan 1` is configured.
When a switch receives an untagged frame on an 802.1Q trunk port, it maps that frame to its own configured native VLAN.
3
Determine the resulting network outcome
The frame sent from VLAN 50 on SW-CORE-01 leaks into VLAN 1 on SW-ACCESS-05.
A native VLAN mismatch across a trunk link causes traffic from one VLAN to bleed into a different VLAN on the remote switch.

Key Concept

802.1Q Native VLAN Tagging & Mismatch Behavior
Estimated Time:1m 30s
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Topics

VLAN Configuration and 802.1Q Trunking

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