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Difficulty: MediumTroubleshooting VLAN, Trunking, and Switching Issues

An infrastructure team is auditing an 802.1Q trunk connection between Switch-1 and Switch-2 on interface FastEthernet 0/24. Syslog on Switch-1 displays `%CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on FastEthernet0/24 (1), with Switch-2 FastEthernet0/24 (30)`. Additionally, hosts assigned to VLAN 30 on Switch-1 cannot reach resources on VLAN 30 attached to Switch-2. A check of Switch-1 interface FastEthernet 0/24 shows that its trunk allowed VLAN list currently includes only VLANs 10 and 20. Which TWO of the following configuration steps must be performed on Switch-1 to resolve the native VLAN warning and allow VLAN 30 traffic to traverse the link? (Select TWO.)

  1. Reconfigure the native VLAN on interface FastEthernet 0/24 of Switch-1 to VLAN 30.Answer
  2. Add VLAN 30 to the list of allowed VLANs on interface FastEthernet 0/24 of Switch-1.Answer
  3. C
    Set the Spanning Tree Protocol priority for VLAN 30 on Switch-1 to the maximum value of 65535.
  4. D
    Set interface FastEthernet 0/24 on Switch-1 to half-duplex mode to allow untagged frames.

Answer

The technician must reconfigure the native VLAN on Switch-1 FastEthernet 0/24 to VLAN 30 and append VLAN 30 to the trunk's allowed VLAN list.
Resolving this issue requires fixing two distinct misconfigurations on Switch-1's trunk port. First, changing the native VLAN to VLAN 30 eliminates the mismatch flagged by CDP. Second, adding VLAN 30 to the trunk's allowed VLAN list enables tagged frames for VLAN 30 to pass between the two switches.

Step-by-Step Solution

1
Analyze the CDP error log for the native VLAN configuration difference.
Switch-1 uses native VLAN 1, while Switch-2 uses native VLAN 30 on interface FastEthernet 0/24.
Matching native VLAN configurations across both ends of an 802.1Q trunk link is required to prevent CDP error logs and untagged traffic leaks.
2
Inspect the allowed VLAN trunk pruning list on Switch-1.
VLAN 30 is absent from Switch-1's allowed list (which only lists VLANs 10 and 20).
Trunk interfaces drop traffic for any VLAN not explicitly included in their allowed VLAN filter.
3
Formulate the required remediation commands for Switch-1 interface FastEthernet 0/24.
Reconfigure the native VLAN to 30 and append VLAN 30 to the allowed list.
Executing both changes resolves the native VLAN mismatch alert and permits VLAN 30 traffic across the trunk link.

Key Concept

802.1Q Trunking Native VLAN Matching and Allowed VLAN List Configuration
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Topics

Troubleshooting VLAN, Trunking, and Switching Issues

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