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Difficulty: MediumInterswitch Connectivity and Trunking (802.1Q)

An administrator on Switch-A attempts to add VLAN 40 to an operational IEEE 802.1Q trunk interface currently forwarding traffic for VLANs 10, 20, and 30. The administrator executes the command `switchport trunk allowed vlan 40` in interface configuration mode. Immediately after, users in VLANs 10, 20, and 30 lose connectivity across the trunk link. Which command should the administrator have executed to add VLAN 40 without disrupting existing VLAN traffic?

  1. switchport trunk allowed vlan add 40Answer
  2. B
    switchport trunk allowed vlan append 40
  3. C
    switchport trunk allowed vlan include 40
  4. D
    switchport trunk allowed vlan modify 40

Answer

The administrator should use the command 'switchport trunk allowed vlan add 40'.
In Cisco IOS, executing `switchport trunk allowed vlan <ids>` overwrites the existing list of allowed VLANs on a trunk interface. To preserve existing allowed VLANs while adding a new VLAN, the `add` keyword must be explicitly included.

Step-by-Step Solution

1
Analyze the impact of the executed command 'switchport trunk allowed vlan 40'.
Executing 'switchport trunk allowed vlan 40' replaces the entire trunk allowed list with only VLAN 40, removing VLANs 10, 20, and 30 from being allowed across the link.
Omitting parameter modifiers causes Cisco IOS to completely overwrite the current allowed VLAN list with the specified list.
2
Determine the correct Cisco IOS command syntax to append a VLAN to an active trunk allowed list.
The correct command syntax is 'switchport trunk allowed vlan add 40'.
The 'add' keyword instructs the switch to append VLAN 40 to the existing list rather than replacing it.

Key Concept

802.1Q Trunk Allowed VLAN Modification
Estimated Time:1m 0s
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Topics

Interswitch Connectivity and Trunking (802.1Q)

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