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Zorluk: OrtaTroubleshooting VLAN, Trunking, and Switching Issues

A network administrator is troubleshooting various Layer 2 switching and VLAN issues across enterprise access switches. Match each observed diagnostic log output or physical interface condition to its most likely underlying root cause.

  • Syslog displays repeated `%CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on GigabitEthernet0/1` notifications.The local trunk interface is configured with native VLAN 10 while the remote trunk interface uses native VLAN 20.
  • Interface counters report high counts of late collisions and FCS errors on a full-duplex configured switch port.The connected device port is forced to half-duplex while the switch port is forced to full-duplex.
  • Traffic for VLAN 50 is dropped across the switch-to-switch uplink despite the 802.1Q trunk state showing active.VLAN 50 has been omitted from the allowed VLAN list configured on the trunk interface.
  • An access port connected to a host continuously transitions into an err-disabled state shortly after link-up.BPDU Guard is enabled on the port and detected incoming Spanning Tree Protocol packets from an unauthorized switch.

Cevap

Each Layer 2 symptom matches its specific root cause: Native VLAN Mismatch log pairs with mismatched native VLAN IDs on opposing trunk ends; Late collisions with full-duplex pair with a duplex mismatch; VLAN traffic dropping across an active trunk pairs with missing VLAN from the allowed list; Err-disabled state on an access port pairs with BPDU Guard detecting unauthorized BPDUs.
Each diagnostic log or interface status correlates directly to its layer 2 switching root cause. CDP native VLAN mismatch indicates unequal native VLAN IDs. Late collisions on full-duplex interfaces indicate a duplex mismatch with a half-duplex partner. Dropped traffic for a single VLAN across a functional trunk indicates an incomplete allowed VLAN list. An err-disabled host port upon link-up indicates BPDU Guard protective action against incoming switch frame BPDUs.

Adım Adım Çözüm

1
Analyze syslog log `%CDP-4-NATIVE_VLAN_MISMATCH`.
CDP detects disparate native VLAN configurations between the local interface and remote neighbor.
802.1Q trunks must agree on the native VLAN number to prevent cross-VLAN traffic leaking.
2
Examine interface counter errors reporting late collisions.
Identifies duplex mismatch where one end is configured full-duplex and the connected end is half-duplex.
Full-duplex ports do not check collision domains, while half-duplex hosts retransmit late after 64 bytes.
3
Evaluate missing VLAN connectivity over active trunk link.
Determines VLAN 50 is omitted from `switchport trunk allowed vlan` configuration.
Trunks prune frames belonging to VLANs not explicitly permitted in their allowed list.
4
Investigate err-disabled port state on host access interface.
Confirms BPDU Guard trigger due to incoming BPDUs from an unauthorized switch or device.
BPDU Guard disables access ports upon BPDU reception to safeguard STP topology integrity.

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

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