A network administrator is diagnosing several Layer 2 switching and trunking issues across a enterprise campus network. Match each observed switch diagnostic symptom or log message on the left to its corresponding underlying root cause on the right.
- Console log repeatedly outputs `%CDP-4-NATIVE_VLAN_MISMATCH` on interface GigabitEthernet0/1.Opposing trunk interfaces are configured with mismatched native VLAN identifiers.
- Interface status reports `FastEthernet0/12 is err-disabled` immediately after an unauthorized switch is attached.BPDU Guard disabled the port upon receiving unexpected Spanning Tree Protocol BPDUs.
- Trunk port allows VLAN 10 traffic across switches, but frames belonging to VLAN 30 are dropped.The required VLAN ID is absent from the trunk interface's allowed VLAN list.
- Switchport statistics show a steady increase in late collisions and CRC error counters during high traffic.A duplex mismatch exists between the switch port and the connected end station.
Answer
Console log %CDP-4-NATIVE_VLAN_MISMATCH matches opposing trunk interfaces configured with mismatched native VLAN identifiers. Interface status err-disabled due to bpduguard matches BPDU Guard disabling the port upon receiving unexpected STP BPDUs. Trunk link dropping VLAN 30 traffic matches the required VLAN ID being absent from the allowed VLAN list. High late collisions and CRC errors match a duplex mismatch between connected devices.
Each diagnostic indicator corresponds to a specific Layer 2 operational fault: CDP explicitly identifies native VLAN discrepancies on trunk links; BPDU Guard disables ports receiving rogue BPDUs; trunk allowed lists filter specific VLAN traffic; and late collisions are the classic hallmark indicator of a half-duplex/full-duplex mismatch.
Step-by-Step Solution
Key Concept
VLAN, Trunking, and Switching Diagnostic Troubleshooting