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Zorluk: KolayTroubleshooting VLAN, Trunking, and Switching Issues

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.

Cevap

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.

Adım Adım Çözüm

1
Analyze the console warning message `%CDP-4-NATIVE_VLAN_MISMATCH`.
Identified that CDP explicitly flags native VLAN mismatches across trunk links.
802.1Q trunks must share identical native VLAN configurations on both connected switch interfaces to avoid untagged frame leakage.
2
Examine the `err-disabled` port state triggered by BPDU receipt.
Linked the state to BPDU Guard enforcement on an access port.
BPDU Guard safeguards network topology by disabling access ports if an incoming Bridge Protocol Data Unit is detected.
3
Evaluate selective VLAN traffic failure on a trunk link.
Determined VLAN 30 is pruned by the allowed VLAN list configuration.
Trunk interfaces only forward traffic for VLANs explicitly allowed in their trunking profile.
4
Inspect interface performance counters showing late collisions.
Associated late collisions and CRC errors with a full/half duplex setting mismatch.
Half-duplex ports listen before transmitting but collide with full-duplex transmissions that occur simultaneously.

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VLAN, Trunking, and Switching Diagnostic Troubleshooting
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Troubleshooting VLAN, Trunking, and Switching Issues

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