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Difficulty: Very hardTroubleshooting VLAN, Trunking, and Switching Issues

A network administrator is troubleshooting an 802.1Q trunk connection between Switch-1 and Switch-2. Switch-1 is configured with a native VLAN of 10 and allows VLANs 10, 20, and 30 across its trunk interface GigabitEthernet0/1. Switch-2 has its GigabitEthernet0/1 trunk interface configured with a native VLAN of 20 and allows only VLANs 10 and 20. Console logs indicate recurring CDP native VLAN mismatch messages, and workstations residing on VLAN 30 attached to Switch-1 are completely unable to communicate with VLAN 30 hosts attached to Switch-2.

Which TWO configuration adjustments must be performed on Switch-2 to resolve the console error logs and restore VLAN 30 trunking communication?

  1. Configure the native VLAN on Switch-2 interface GigabitEthernet0/1 to VLAN 10.Answer
  2. Add VLAN 30 to the allowed VLAN list on Switch-2 interface GigabitEthernet0/1.Answer
  3. C
    Lower the Spanning Tree Protocol (STP) priority value on Switch-2 for VLAN 30 to force it to become the STP root bridge.
  4. D
    Change the operating duplex mode on Switch-2 interface GigabitEthernet0/1 from full-duplex to half-duplex.

Answer

To resolve the issues, Switch-2 must have its trunk native VLAN updated to VLAN 10 to match Switch-1, and VLAN 30 must be explicitly added to the trunk port's allowed VLAN list.
The correct adjustments require aligning the native VLAN ID across both ends of the trunk (setting Switch-2 native VLAN to 10) to clear the CDP native VLAN mismatch log, and expanding Switch-2's allowed VLAN trunk list to include VLAN 30 so that tagged frames for VLAN 30 can be received and forwarded.

Step-by-Step Solution

1
Analyze the native VLAN mismatch error between Switch-1 and Switch-2.
Switch-1 uses VLAN 10 as native, while Switch-2 uses VLAN 20. 802.1Q trunks send native VLAN frames untagged, causing cross-VLAN traffic leakage and log errors.
Both ends of an 802.1Q trunk must agree on the native VLAN ID so untagged frames are properly mapped to the identical broadcast domain on both switches.
2
Analyze the VLAN 30 connectivity failure across the trunk.
Switch-2 allowed list only includes VLANs 10 and 20. VLAN 30 frames sent by Switch-1 are dropped at Switch-2's trunk ingress.
Trunk interfaces prune traffic for VLANs not explicitly included in their allowed VLAN list.
3
Determine the required remedial commands on Switch-2.
Reconfigure Switch-2 GigabitEthernet0/1 to use native VLAN 10 and add VLAN 30 to the trunk allowed VLAN list.
These two steps directly clear the console error and allow VLAN 30 tagged traffic to pass.

Key Concept

802.1Q Trunking Parameters & Native VLAN / Allowed VLAN Troubleshooting
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Topics

Troubleshooting VLAN, Trunking, and Switching Issues

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