What is the root cause of the problem?

The network manager has requested that several new VLANs (VLAN 10, 20, and 30) are allowed to traverse the switch trunk interface. After the command switchport trunk allowed vlan 10,20,30 is issued, all other existing VLANs no longer pass traffic over the trunk.

What is the root cause of the problem?
A . The command effectively removed all other working VLANs and replaced them with the new VLANs.
B . VTP pruning removed all unused VLANs.
C . ISL was unable to encapsulate more than the already permitted VLANs across the trunk.
D . Allowing additional VLANs across the trunk introduced a loop in the network.

Answer: A

Explanation:

The "switchport trunk allowed vlan" command will only allow the specified VLANs, and overwrite any others that were previously defined. You would also need to explicitly allow the other working VLANs to this configuration command, or use the "issue the switchport trunk allowed vlan add vlan-list" command instead to add these 3 VLANS to the other defined allowed VLANs. Reference: https:// supportforums.cisco.com/document/11836/how-define-vlans-allowed-trunk- link