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Difficulty: HardVLAN Configuration and 802.1Q Trunking

A network administrator is configuring a GigabitEthernet 0/1 trunk link on switch SW-BuildingA to connect to SW-Core. The link is intended to carry traffic for VLAN 10 (Data), VLAN 20 (Voice), and VLAN 99 (Management). VLAN 99 is configured as the native VLAN on both switches. The administrator executes the command `switchport trunk allowed vlan 10,20` on interface GigabitEthernet 0/1. Immediately after executing the command, remote management access to SW-BuildingA via VLAN 99 drops. Which of the following best explains why management traffic is no longer traversing the trunk link?

  1. The explicit allowed VLAN list replaces the default allowed range and excludes VLAN 99, preventing its frames from traversing the trunk regardless of native VLAN designation.Answer
  2. B
    Native VLAN traffic is automatically tagged with 802.1Q headers when allowed VLAN lists are modified, causing a tag mismatch at the receiving switch.
  3. C
    Untagged native VLAN frames cannot cross an 802.1Q trunk unless inter-VLAN routing is active on the physical interface.
  4. D
    Applying an allowed VLAN list automatically reverts the switchport operational mode from trunk to access, disabling multi-VLAN support.

Answer

The explicit allowed VLAN list replaces the default allowed range and excludes VLAN 99, preventing its frames from traversing the trunk regardless of native VLAN designation.
On an 802.1Q trunk, the allowed VLAN list controls which VLANs can send and receive frames across the link. When `switchport trunk allowed vlan 10,20` is executed without the `add` keyword, it replaces the entire allowed list with only VLANs 10 and 20. Because VLAN 99 is excluded from this list, all frames associated with VLAN 99 are blocked, causing loss of management access.

Step-by-Step Solution

1
Analyze the command executed on the trunk interface.
The command `switchport trunk allowed vlan 10,20` sets the active allowed list on the interface strictly to VLANs 10 and 20.
By default, trunks allow all VLANs (1-4094). Specifying a list parameter without the `add` keyword overwrites the existing allowed list.
2
Evaluate the interaction between native VLAN configuration and allowed VLAN lists.
VLAN 99 is designated as the native VLAN, but it is not present in the allowed list (10, 20).
An 802.1Q trunk will only forward frames for VLANs that are explicitly present in its allowed VLAN list, even if that VLAN is configured as the native VLAN.
3
Determine the impact on management traffic.
Management traffic belonging to VLAN 99 is pruned and dropped at interface GigabitEthernet 0/1.
Because VLAN 99 is excluded from the allowed list, remote management traffic sent over VLAN 99 cannot traverse the link.

Key Concept

802.1Q Trunk Allowed VLAN Filtering
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Topics

VLAN Configuration and 802.1Q Trunking

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