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Zorluk: Çok zorTroubleshooting VLAN, Trunking, and Switching Issues

A network administrator is reviewing switch console logs and interface statistics to diagnose several Layer 2 performance and connectivity issues across the enterprise network. Match each specific switch error output or diagnostic symptom on the left with its primary underlying root cause on the right.

  • Interface statistics show a rapidly increasing count of late collisions and FCS errors during peak traffic load.Speed/Duplex mismatch where one endpoint is set to full-duplex and the connecting peer operates in half-duplex.
  • Console log persistently reports %CDP-4-NATIVE_VLAN_MISMATCH on inter-switch trunk link interface GigabitEthernet0/1.Mismatched native VLAN ID configuration on opposing ends of an 802.1Q trunk link.
  • Access port interface GigabitEthernet0/10 enters an err-disabled state with log %SPANTREE-2-BLOCK_BPDUGUARD.Reception of Spanning Tree Protocol BPDUs on an access port configured with BPDU Guard.
  • Trunk port successfully forwards frames for VLAN 10 and VLAN 20, but drops all frames for VLAN 30 despite VLAN 30 existing in the active VLAN database.VLAN 30 is explicitly omitted from the trunk interface allowed VLAN list configuration.

Cevap

Late collisions and FCS errors match with Speed/Duplex mismatch. CDP native VLAN mismatch logs match with Mismatched native VLAN configuration across 802.1Q trunk endpoints. BPDU Guard error-disabled log matches with Reception of BPDUs on an edge access port. Traffic drop for active VLAN 30 on trunk matches with VLAN 30 omitted from the trunk allowed VLAN list.
Each diagnostic log message and port metric directly aligns with a fundamental Layer 2 misconfiguration: late collisions indicate a duplex mismatch where half-duplex senses collision after slot time; CDP warnings isolate mismatched native VLAN settings; BPDU Guard messages indicate unexpected STP frames on edge ports; and selective VLAN traffic loss on trunks points to allowed-list misconfigurations.

Adım Adım Çözüm

1
Analyze interface collision and error metrics.
Identify late collisions as the classic indicator of a duplex mismatch between full-duplex and half-duplex endpoints.
The full-duplex endpoint transmits at any time, causing the half-duplex endpoint to detect collisions after sending the preamble and slot time (64 bytes).
2
Inspect CDP diagnostic log warnings.
Determine that %CDP-4-NATIVE_VLAN_MISMATCH directly indicates mismatched untagged VLAN IDs on trunk link ends.
802.1Q trunks expect identical native VLAN IDs on both ends to ensure untagged control and data frames route to the correct broadcast domain.
3
Evaluate Spanning Tree Protocol port security features.
Recognize that receiving BPDUs on a BPDU Guard enabled port triggers the %SPANTREE-2-BLOCK_BPDUGUARD error and disables the port.
BPDU Guard prevents rogue switches or unauthorized bridge connections from altering the STP topology via designated edge access ports.
4
Verify trunking allowed-list parameters against active VLANs.
Establish that omitting a VLAN from `switchport trunk allowed vlan` causes the switch to filter and drop frames for that VLAN on the trunk interface.
Global VLAN database existence is necessary but insufficient; the VLAN must also be permitted on the specific trunk interface allowed list.

Anahtar Kavram

Troubleshooting Layer 2 Switching, Trunking, Duplex, and Spanning Tree Operational Issues
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Troubleshooting VLAN, Trunking, and Switching Issues

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