350-401 Question 121
Select 4An enterprise network administrator reports that a VLAN created on a switch is not propagating to other switches in the network. Upon investigation, it is discovered that the switches are connected via trunk links. Which of the following could be the reason for this issue?
- A
The VLAN was not added to the allowed VLAN list on the trunk port.
- B
The trunk port is configured with the 'switchport mode access' command.
- C
The VTP mode on the switch is set to 'Transparent'.
- D
The VLAN was not created on all switches in the network.
- E
The trunk port is using an unsupported encapsulation protocol.
Show answer and explanation
Correct answers: A, B, C, E
Explanation
For VLAN propagation to work properly across switches, several conditions must be met. The VLAN must be allowed on trunk links, the trunk ports must be correctly configured, the VTP mode should support VLAN propagation (e.g., not set to Transparent), and the trunk encapsulation protocol should match between switches. If any of these conditions are not met, VLAN propagation issues can occur. Manually creating the VLAN on all switches is not required if VTP is functioning correctly.
- A. Correct.
If the VLAN is not added to the allowed VLAN list on a trunk port, it will not propagate to other switches. Trunk ports only forward traffic for VLANs that are explicitly allowed.
- B. Correct.
If the trunk port is configured as an access port using 'switchport mode access', it will not operate as a trunk and will not forward VLAN information to other switches.
- C. Correct.
In VTP Transparent mode, VLAN information is not propagated to other switches, which could cause the VLAN to remain local to the switch.
- D. Incorrect.
VLANs do not need to be manually created on every switch if VTP is properly functioning. VTP can propagate VLAN information automatically.
- E. Correct.
If the trunk port is using an unsupported encapsulation protocol (e.g., mismatch between ISL and 802.1Q), VLAN information and traffic will not propagate correctly.