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Difficulty: Very hardVLAN Configuration and 802.1Q Trunking

A network engineer is troubleshooting interconnectivity between two switches, Switch-Alpha and Switch-Beta, over a dedicated GigabitEthernet link. The switchport configuration for interface GigabitEthernet1/0/24 on both switches is shown below:

Switch-Alpha:
interface GigabitEthernet1/0/24
switchport mode trunk
switchport trunk native vlan 50
switchport trunk allowed vlan 10,20,50

Switch-Beta:
interface GigabitEthernet1/0/24
switchport mode trunk
switchport trunk native vlan 1
switchport trunk allowed vlan 10,20,50

A host in VLAN 10 on Switch-Alpha sends data to a host in VLAN 10 on Switch-Beta. Simultaneously, an untagged broadcast frame is generated by a device in VLAN 50 on Switch-Alpha and transmitted across the link.

Which statement accurately describes how these frames are handled across the trunk link?

  1. Tagged frames belonging to VLAN 10 cross the trunk link normally, while untagged frames sent from VLAN 50 on Switch-Alpha are placed into VLAN 1 upon arrival at Switch-Beta.Answer
  2. B
    All frame transmissions across GigabitEthernet1/0/24 are immediately blocked because detecting a native VLAN mismatch forces the trunk interface into an err-disabled state.
  3. C
    Untagged frames from VLAN 50 on Switch-Alpha are automatically routed directly into VLAN 10 by Switch-Beta without traversing a Layer 3 routing engine.
  4. D
    VLAN 10 frames are stripped of their 802.1Q headers at Switch-Alpha because VLAN 50 is designated as the native VLAN on that egress interface.

Answer

Tagged frames belonging to VLAN 10 cross the trunk link normally, while untagged frames sent from VLAN 50 on Switch-Alpha are placed into VLAN 1 upon arrival at Switch-Beta.
Under 802.1Q encapsulation, frames belonging to allowed tagged VLANs (such as VLAN 10) are explicitly tagged and delivered correctly across the trunk. However, frames originating from VLAN 50 on Switch-Alpha are egressed untagged because VLAN 50 is Switch-Alpha's native VLAN. When Switch-Beta receives these untagged frames, it classifies them according to its own native VLAN setting (VLAN 1), leading to native VLAN traffic leakage between VLAN 50 and VLAN 1.

Step-by-Step Solution

1
Analyze the handling of tagged VLAN traffic across the trunk link.
VLAN 10 is configured in the allowed list on both ends of the trunk. Switch-Alpha adds an 802.1Q tag (VLAN ID 10) to the frame. Switch-Beta reads the tag, accepts the frame, and forwards it to VLAN 10.
802.1Q trunks maintain tag headers for non-native allowed VLANs regardless of native VLAN configuration.
2
Analyze the egress behavior for VLAN 50 on Switch-Alpha.
Because VLAN 50 is Switch-Alpha's configured native VLAN on GigabitEthernet1/0/24, frames originating in VLAN 50 are transmitted out the trunk port untagged (without an 802.1Q header).
By default in IEEE 802.1Q specification, traffic matching the local interface's native VLAN is sent unencapsulated.
3
Analyze the ingress behavior for untagged frames arriving at Switch-Beta.
Switch-Beta receives the untagged frame and associates it with its own configured native VLAN, which is VLAN 1.
When a trunk port receives an untagged frame, it automatically assigns that frame to whichever VLAN is defined as native on its local interface.

Key Concept

802.1Q Native VLAN Ingress/Egress Behavior during a Trunk Mismatch
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Topics

VLAN Configuration and 802.1Q Trunking

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