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Difficulty: HardTroubleshooting VLAN, Trunking, and Switching Issues

A network engineer is diagnosing intermittent performance issues and log warnings on an inter-switch trunk link connecting Switch-Alpha (GigabitEthernet0/1) to Switch-Beta (GigabitEthernet0/1). The console on Switch-Alpha displays `%CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on GigabitEthernet0/1 (10), with Switch-Beta GigabitEthernet0/1 (20)`. Concurrently, interface statistics for GigabitEthernet0/1 on Switch-Alpha show a steadily increasing count of late collisions and Frame Check Sequence (FCS) errors. Which TWO of the following root causes are directly responsible for the observed log messages and interface error counters?

  1. Inconsistent 802.1Q native VLAN configurations on the opposing ends of the trunk link.Answer
  2. A duplex mismatch between the connected GigabitEthernet interfaces on Switch-Alpha and Switch-Beta.Answer
  3. C
    Switch-Alpha being assigned a higher numerical Spanning Tree Protocol (STP) bridge priority, putting GigabitEthernet0/1 into an STP blocking state.
  4. D
    An absence of a Layer 3 router on the trunk link to facilitate frame delivery for devices sharing identical VLAN tags across switches.

Answer

The two correct root causes are inconsistent 802.1Q native VLAN configurations across the trunk link and a duplex mismatch between the connected interfaces.
The correct selections identify the two distinct issues present: the explicit Cisco Discovery Protocol log indicates an 802.1Q native VLAN ID mismatch between the switch ports, and the presence of late collisions with FCS errors points directly to a speed/duplex negotiation mismatch.

Step-by-Step Solution

1
Analyze the CDP warning log message
The message %CDP-4-NATIVE_VLAN_MISMATCH confirms that Switch-Alpha uses native VLAN 10 while Switch-Beta uses native VLAN 20 on their respective trunk ports.
802.1Q trunks must share the same native VLAN ID on both sides so untagged control and data frames are correctly interpreted and isolated.
2
Analyze the interface error metrics (late collisions and FCS errors)
Late collisions occur when an interface operating in half-duplex detects a collision after transmit window timing has elapsed, caused by the opposite end transmitting simultaneously in full-duplex mode.
In a full-duplex to half-duplex mismatch, the full-duplex side ignores carrier sense and transmits at will, resulting in collisions during transmission on the half-duplex side.
3
Evaluate remaining options against observed symptoms
STP bridge priority misconfigurations affect root election and topology blocking but do not cause physical/data-link layer framing or collision errors. Layer 3 routers are only needed for inter-VLAN routing, not intra-VLAN frame forwarding across Layer 2 trunks.
Eliminating invalid causes confirms native VLAN mismatch and duplex mismatch as the two independent issues.

Key Concept

Troubleshooting Layer 2 trunk misconfigurations (Native VLAN mismatches) and physical-layer interface mismatches (Speed/Duplex mismatch indicators).
Estimated Time:2m 0s
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Topics

Troubleshooting VLAN, Trunking, and Switching Issues

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