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Difficulty: HardTroubleshooting VLAN, Trunking, and Switching Issues

A network engineer is analyzing several Layer 2 switching and trunking issues across an enterprise network. Match each observed switch diagnostic indicator or log message on the left to its underlying root cause on the right.

  • Console log repeatedly displays %CDP-4-NATIVE_VLAN_MISMATCH while untagged management frames spill into inappropriate VLANs across the inter-switch link.Mismatched native VLAN IDs configured on opposing ends of the 802.1Q trunk link.
  • Interface statistics report a high count of late collisions, duplex mismatch symptoms, and frame check sequence (FCS) errors on a link set to full-duplex.Speed/duplex autonegotiation failure where one side is hardcoded to full-duplex while the opposing side defaults to half-duplex.
  • A non-root switch port unexpectedly transitions to an STP Blocking state after receiving BPDUs with a bridge priority of 4096 from an unauthorized access switch.STP root bridge election hijacking caused by a lower bridge priority advertised by a rogue or newly introduced switch.
  • Tagged traffic for VLAN 50 is dropped across an 802.1Q trunk interface, while untagged native traffic and other tagged VLANs transit normally.Omission of VLAN 50 from the switchport trunk allowed vlan configuration parameter.

Answer

Each diagnostic indicator corresponds directly to a specific Layer 2 misconfiguration: Native VLAN mismatch CDP logs indicate differing native VLAN configurations across the trunk; late collisions indicate an autonegotiation mismatch resulting in half-duplex operation on one peer; unexpected STP blocking indicates a superior BPDU causing root bridge re-election; and specific tagged VLAN traffic dropping indicates pruning via the trunk allowed VLAN list.
The correct pairings accurately associate specific diagnostic output to their root causes: %CDP-4-NATIVE_VLAN_MISMATCH directly indicates conflicting native VLAN configurations on trunk endpoints; late collision counters indicate a duplex mismatch where one end operates half-duplex; unexpected STP blocking states result from receiving superior BPDUs with a lower bridge priority; and isolated VLAN traffic dropping across a trunk indicates an incomplete trunk allowed VLAN list.

Step-by-Step Solution

1
Analyze the CDP log message regarding native VLAN mismatch.
Identify that 802.1Q trunking sends native VLAN traffic untagged. If native VLAN IDs differ on opposite sides, CDP logs an explicit mismatch error.
Matching native VLAN configurations are required on both ends of an 802.1Q trunk for proper untagged frame processing.
2
Examine interface collision counters and error types.
Recognize that late collisions (collisions occurring after transmitting the first 512 bits / 64 bytes of a frame) are a definitive diagnostic indicator of a duplex mismatch, typically caused when one port is hardcoded to full-duplex and the connected port falls back to half-duplex autonegotiation.
Full-duplex interfaces do not test for carrier sense or collisions, transmitting while the half-duplex side is mid-frame.
3
Evaluate Spanning Tree Protocol (STP) state changes.
Determine that receiving a BPDU with a priority value of 4096 (lower than the default 32768) represents a superior BPDU, forcing STP topology re-convergence and altering port blocking states.
STP elects the switch with the lowest Bridge ID (Priority + MAC address) as the root bridge.
4
Correlate selective VLAN dropping on trunk links.
Confirm that when specific VLAN traffic is dropped while other VLANs succeed, the affected VLAN ID has been omitted from the allowed list configured on the trunk port.
Trunk interfaces only forward traffic for VLANs explicitly permitted in their allowed VLAN list.

Key Concept

Layer 2 VLAN, Trunking, and STP Troubleshooting Indicators
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Topics

Troubleshooting VLAN, Trunking, and Switching Issues

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