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Difficulty: MediumVLAN Configuration and 802.1Q Trunking

A network administrator attempts to add VLAN 30 to an active 802.1Q trunk line between two distribution switches that currently carries VLAN 10 and VLAN 20. The administrator enters the following command on interface GigabitEthernet1/0/1:

`SW-Dist1(config-if)# switchport trunk allowed vlan 30`

Immediately after executing this command, users on VLAN 10 and VLAN 20 report loss of connectivity across the trunk link. Which of the following best explains why this connectivity issue occurred?

  1. The command replaced the existing allowed VLAN list with only VLAN 30 rather than appending it to the list.Answer
  2. B
    VLAN 30 became the new native VLAN on the interface, causing all untagged traffic to be dropped automatically.
  3. C
    VLAN 10 and VLAN 20 were converted from isolated broadcast domains into access mode ports on GigabitEthernet1/0/1.
  4. D
    The trunk link disabled IEEE 802.1Q frame tagging and reverted to unencapsulated Ethernet frame transmission.

Answer

The command executed overwrote the entire allowed VLAN list on the trunk interface with VLAN 30, thereby pruning VLAN 10 and VLAN 20 from traversing the link.
When executing `switchport trunk allowed vlan <vlan-list>` without using the `add` keyword, the switch replaces the entire set of allowed VLANs on the trunk with only the specified VLAN(s). As a result, VLAN 10 and VLAN 20 were pruned from GigabitEthernet1/0/1, causing cross-switch traffic for those VLANs to be dropped.

Step-by-Step Solution

1
Analyze the CLI command executed on the switch interface.
Identified the command as `switchport trunk allowed vlan 30` without the `add` parameter.
In Cisco IOS and standard network switch syntax, `switchport trunk allowed vlan <vlan-id>` sets an explicit allowed list, replacing any previously permitted VLANs.
2
Evaluate the impact on active VLAN traffic traversing the trunk.
VLAN 10 and VLAN 20 are immediately removed from the trunk's allowed list.
Frames belonging to VLAN 10 and VLAN 20 are dropped at egress/ingress on GigabitEthernet1/0/1 because those VLAN IDs are no longer permitted on the trunk interface.
3
Determine the proper command syntax to prevent this issue.
The administrator should have executed `switchport trunk allowed vlan add 30`.
Using the `add` parameter appends the specified VLAN ID to the existing list of permitted VLANs without clearing current entries.

Key Concept

802.1Q Trunk Allowed VLAN List Overwrite vs. Append
Estimated Time:1m 30s
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Topics

VLAN Configuration and 802.1Q Trunking

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