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Zorluk: ZorTroubleshooting VLAN, Trunking, and Switching Issues

A network technician is investigating poor application throughput and significant packet loss on an enterprise host connected to switch interface FastEthernet 0/12. Output from the switch console reveals a high count of late collisions, frame check sequence (FCS) errors, and deferred transmissions on the port. A review of device settings shows that the host network interface card (NIC) was manually forced to 100 Mbps Full Duplex, while the connected switchport is configured with default auto-negotiation parameters. Which of the following best explains the root cause of this performance issue?

  1. The switchport failed auto-negotiation, defaulting to half-duplex mode and creating a duplex mismatch with the full-duplex host.Cevap
  2. B
    The switchport's 802.1Q native VLAN is mismatched with the untagged management VLAN assigned to the host NIC.
  3. C
    The switchport is experiencing Spanning Tree Protocol (STP) topology change notifications due to an invalid STP root bridge priority setting.
  4. D
    The host is attempting to communicate with devices on another VLAN without an operational Layer 3 routing interface.

Cevap

The switchport failed auto-negotiation, defaulting to half-duplex mode and creating a duplex mismatch with the full-duplex host.
When an Ethernet interface configured for auto-negotiation connects to a link partner configured for forced (manual) full-duplex, auto-negotiation fails to detect the duplex mode. By IEEE standard convention, the auto-negotiation interface sets its speed using parallel detection but defaults its duplex setting to half-duplex. This mismatch causes the full-duplex side to transmit at will while the half-duplex side detects collisions during transmission after the standard 64-byte slot time (late collisions), leading to FCS errors and severe throughput degradation.

Adım Adım Çözüm

1
Analyze the interface statistics and configuration settings.
The switch interface records late collisions and FCS errors, while the host NIC is set to forced 100 Mbps Full Duplex and the switch port is set to Auto-Negotiation.
Late collisions are a signature symptom of a duplex mismatch on an Ethernet segment.
2
Apply Ethernet auto-negotiation standard behavior rules (IEEE 802.3).
Because the host NIC does not transmit auto-negotiation pulses, the switch cannot negotiate duplex and falls back to half-duplex mode.
Standard Ethernet auto-negotiation defaults to half-duplex when speed can be sensed via parallel detection but duplex cannot be negotiated.
3
Determine the impact of half-duplex operating against full-duplex.
The host transmits whenever it has data ready (full-duplex), while the switch listens before transmitting and senses collisions after the 64-byte window (late collisions), causing corrupted frames and FCS errors.
This confirms a duplex mismatch as the root cause.

Anahtar Kavram

Duplex Mismatch Troubleshooting & Auto-Negotiation Fallback Rules
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Konular

Troubleshooting VLAN, Trunking, and Switching Issues

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