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Difficulty: MediumVLAN Configuration and 802.1Q Trunking

A network administrator is inspecting an 802.1Q trunk link configured between Switch-A and Switch-B. Interface GigabitEthernet0/1 on Switch-A is configured with `switchport trunk native vlan 10`. On Switch-B, interface GigabitEthernet0/1 is configured with `switchport trunk native vlan 50`. When Switch-A transmits untagged Ethernet frames across this link to Switch-B, which of the following outcomes will occur?

  1. Switch-B will accept the untagged frames and associate them with VLAN 50.Answer
  2. B
    The untagged frames will be redirected to a Layer 3 router to resolve the broadcast domain conflict.
  3. C
    Switch-B will automatically drop all incoming untagged frames until 802.1Q tagging is enabled for all VLANs.
  4. D
    Switch-B will prepend a VLAN 10 tag to the frames upon ingress based on Switch-A's trunk configuration.

Answer

Switch-B will accept the untagged frames and associate them with VLAN 50.
Under 802.1Q trunking, traffic belonging to the configured native VLAN is sent untagged across the link. Switch-A sends VLAN 10 traffic untagged because VLAN 10 is its native VLAN. Switch-B receives the untagged traffic and assigns it to its own local native VLAN, which is VLAN 50. This results in untagged frames leaking from VLAN 10 on Switch-A into VLAN 50 on Switch-B.

Step-by-Step Solution

1
Determine frame transmission behavior on Switch-A
Frames belonging to VLAN 10 on Switch-A match its native VLAN setting (`vlan 10`). Consequently, Switch-A transmits these frames across the 802.1Q trunk link without an explicit 4-byte 802.1Q header tag.
802.1Q trunking specifies that frames on the native VLAN are forwarded untagged.
2
Determine frame reception behavior on Switch-B
Switch-B receives untagged frames on interface GigabitEthernet0/1. Switch-B checks its local port configuration, identifies its native VLAN as VLAN 50, and places the incoming untagged traffic into VLAN 50.
Switches place all untagged ingress frames on an 802.1Q trunk port into the native VLAN configured on that local port.
3
Evaluate the net impact of the configuration
Traffic originating on VLAN 10 on Switch-A improperly leaks into VLAN 50 on Switch-B, creating a security issue and native VLAN mismatch condition.
Mismatched native VLAN IDs across a trunk link cause untagged frame cross-talk between different VLANs.

Key Concept

802.1Q Native VLAN Untagged Frame Processing and Mismatch Behavior
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Topics

VLAN Configuration and 802.1Q Trunking

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