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Difficulty: HardVLAN Configuration and 802.1Q Trunking

An enterprise network engineer is troubleshooting interconnectivity between two switches, SW-East and SW-West. The switches are connected via a direct fiber link on interface GigabitEthernet1/0/24.

The interface configuration on SW-East is:
text
interface GigabitEthernet1/0/24
switchport mode trunk
switchport trunk native vlan 100
switchport trunk allowed vlan 10,20,30

The interface configuration on SW-West is:
text
interface GigabitEthernet1/0/24
switchport mode trunk
switchport trunk native vlan 100
switchport trunk allowed vlan 10,20,100

Workstations in VLAN 30 connected to SW-East cannot reach resources in VLAN 30 connected to SW-West, while devices on VLAN 10 and VLAN 20 function normally across both switches. System diagnostic logs report no native VLAN mismatch errors.

Which of the following best explains why traffic for VLAN 30 fails to traverse the trunk link?

  1. VLAN 30 is omitted from the allowed VLAN list on SW-West, causing ingress frames tagged with VLAN 30 to be dropped by SW-West.Answer
  2. B
    Because native VLAN 100 is explicitly configured, all untagged traffic is automatically mapped to VLAN 30 on ingress, starving tagged VLAN 30 frames.
  3. C
    Inter-switch communication between identical non-native VLANs requires an intermediary Layer 3 router when custom native VLAN IDs are assigned.
  4. D
    Spanning Tree Protocol automatically puts VLAN 30 into a blocking state on SW-East because the native VLAN ID is numerically higher than 30.

Answer

VLAN 30 is omitted from the allowed VLAN list on SW-West, causing ingress frames tagged with VLAN 30 to be dropped by SW-West.
For an 802.1Q trunk to successfully pass traffic for a given VLAN in both directions, that VLAN must be configured in the allowed VLAN list on both switch interfaces. SW-East allows VLAN 30, but SW-West explicitly restricts allowed traffic to VLANs 10, 20, and 100. Consequently, frames tagged with VLAN 30 sent from SW-East are discarded upon ingress at SW-West's interface.

Step-by-Step Solution

1
Examine the trunk interface configuration on both switches
SW-East allows VLANs 10, 20, and 30. SW-West allows VLANs 10, 20, and 100.
802.1Q trunks only forward frames for VLANs explicitly included in their trunk allowed lists.
2
Trace frame transmission for VLAN 30 from SW-East to SW-West
SW-East inserts an 802.1Q tag for VLAN 30 and transmits the frame across GigabitEthernet1/0/24.
VLAN 30 is not the native VLAN (100), so frames leaving SW-East carry a 4-byte 802.1Q header with VLAN ID 30.
3
Evaluate frame processing upon arrival at SW-West interface GigabitEthernet1/0/24
SW-West checks the allowed VLAN list (10, 20, 100) and drops the VLAN 30 tagged frame.
Since VLAN 30 is missing from SW-West's allowed VLAN list, ingress filtering drops the tagged frame.

Key Concept

802.1Q Trunk Allowed VLAN List Pruning and Filtering
Estimated Time:2m 0s
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Topics

VLAN Configuration and 802.1Q Trunking

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