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Difficulty: MediumVLAN Configuration and 802.1Q Trunking

A network administrator configures interface GigabitEthernet0/2 on switch SW-CORE-01 as an 802.1Q trunk with the following commands:

text
interface GigabitEthernet0/2
switchport mode trunk
switchport trunk native vlan 50
switchport trunk allowed vlan 10,20,30

Which of the following describes how switch SW-CORE-01 will handle an incoming untagged Ethernet frame received on interface GigabitEthernet0/2?

  1. The switch drops the incoming untagged frame because native VLAN 50 is not present in the trunk's allowed VLAN list.Answer
  2. B
    The switch automatically tags the incoming untagged frame with VLAN 10 because VLAN 10 is the lowest numerical allowed VLAN.
  3. C
    The switch accepts the frame into VLAN 50 and automatically forwards it to hosts in VLAN 10 without requiring Layer 3 routing.
  4. D
    The switch encapsulates the frame with an 802.1Q tag for VLAN 50 before sending it across the trunk link.

Answer

The switch drops the incoming untagged frame because native VLAN 50 is not present in the trunk's allowed VLAN list.
On an IEEE 802.1Q trunk link, any untagged Ethernet frame received by a switchport is associated with the native VLAN configured on that port (in this case, VLAN 50). However, the `switchport trunk allowed vlan` command acts as a strict VLAN filter on both ingress and egress. Because VLAN 50 is omitted from the allowed VLAN list (`10,20,30`), the switch filters out VLAN 50 traffic, causing the incoming untagged frame to be dropped immediately.

Step-by-Step Solution

1
Identify how untagged frames are classified on an 802.1Q trunk.
The frame is mapped to the port's configured native VLAN, which is VLAN 50.
IEEE 802.1Q trunking assigns untagged incoming frames to the native VLAN designated on that switchport.
2
Evaluate the allowed VLAN list configured on the interface.
The allowed list is explicitly constrained to VLANs 10, 20, and 30.
The command `switchport trunk allowed vlan 10,20,30` restricts ingress and egress traffic on the trunk to only those specified VLANs.
3
Determine the switch processing decision for VLAN 50 traffic.
The frame is dropped at the ingress interface.
If the native VLAN is excluded from the allowed VLAN pruning list, ingress untagged frames assigned to that native VLAN cannot traverse the trunk and are discarded.

Key Concept

802.1Q Native VLAN Ingress Filtering & Allowed VLAN Interaction
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Topics

VLAN Configuration and 802.1Q Trunking

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