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Zorluk: ZorTroubleshooting VLAN, Trunking, and Switching Issues

A network engineer is troubleshooting several Layer 2 connectivity issues across enterprise switch stacks. Match each observed diagnostic log output or interface symptom on the left with its correct underlying root cause on the right.

  • Interface log repeatedly records '%CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on GigabitEthernet0/1 (10), with Switch-B GigabitEthernet0/1 (50)'.802.1Q native VLAN misconfiguration between interconnected trunk ports causing untagged frame mishandling.
  • Interface statistics on a 1 Gbps link show a rapidly incrementing counter for late collisions and FCS errors during high-throughput file transfers.Duplex mismatch caused by one side operating in half-duplex while the opposing side is forced to full-duplex.
  • Console outputs '%SPANTREE-2-BLOCK_BPDUGUARD: Received BPDU on port GigabitEthernet0/12 with BPDU Guard enabled' and transitions port to err-disabled.An unauthorized switch or rogue device sending Spanning Tree Protocol packets was connected to an edge access port.
  • Trunk interface shows line protocol UP, but host traffic tagged with 802.1Q VLAN 40 fails to traverse the link while VLAN 10 traffic passes cleanly.VLAN 40 is missing from the trunk port's allowed VLAN list or has been manually pruned on the trunk interface.

Cevap

Each diagnostic symptom matches its corresponding Layer 2 root cause: CDP native VLAN mismatch log corresponds to native VLAN misconfiguration; late collisions and FCS errors correspond to a duplex mismatch; BPDU Guard err-disable log corresponds to an unauthorized switch sending BPDUs on an access port; and single VLAN loss over an operational trunk corresponds to VLAN exclusion from the allowed trunk list.
Each diagnostic indicator directly maps to a distinct Layer 2 misconfiguration: CDP warning logs explicitly indicate native VLAN mismatches; late collision counters signify half-duplex/full-duplex operation mismatch; BPDU Guard shutdown indicates rogue switch detection on an access port; and single-VLAN failure on an active trunk points to allowed-list pruning.

Adım Adım Çözüm

1
Analyze the CDP mismatch log message.
CDP explicitly identifies that local interface GigabitEthernet0/1 expects native VLAN 10 while Switch-B expects native VLAN 50.
Mismatched native VLANs prevent proper untagged frame processing and generate continuous CDP warnings.
2
Examine the late collisions and FCS error counters on the interface.
Late collisions indicate frames were transmitted while receiving data after 512 bit times (64 bytes), characteristic of half-duplex CSMA/CD operation conflicting with full-duplex.
When one end is set to full-duplex, it transmits regardless of incoming signals, causing the half-duplex end to experience late collisions.
3
Evaluate the BPDU Guard log and port state change.
BPDU Guard is designed to protect access ports configured with PortFast from unauthorized STP topology change messages.
Receiving a BPDU on a BPDU Guard-enabled port forces the switch to err-disable the port to prevent potential switching loops.
4
Diagnose selective VLAN loss across an active trunk link.
Since the link protocol is UP and VLAN 10 functions, trunk encapsulation and physical layer are operational, isolating the issue to VLAN 40 configuration.
A VLAN allowed-list restriction or VTP pruning specific to VLAN 40 blocks that single VLAN while permitting others.

Anahtar Kavram

Layer 2 Switching and Trunking Troubleshooting Diagnostics
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Troubleshooting VLAN, Trunking, and Switching Issues

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