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Zorluk: Çok zorTroubleshooting VLAN, Trunking, and Switching Issues

A network administrator is investigating high latency and severe throughput degradation on a critical database server connected to switch port FastEthernet0/12. An inspection of the switch port statistics shows a high count of late collisions, alignment errors, and frame check sequence (FCS) errors on FastEthernet0/12 during heavy traffic periods. The switch interface is set to default auto-negotiation settings, whereas the server's network interface card (NIC) was recently manually forced to 100 Mbps Full Duplex. Which of the following is the most likely root cause of the late collisions recorded on the switch port?

  1. The switch interface defaulted to half-duplex operation after failing auto-negotiation with the forced full-duplex server NIC.Cevap
  2. B
    The switch port and server NIC are configured with mismatching native VLAN tags, causing untagged frames to be dropped as late collisions.
  3. C
    The server and switch port are assigned to different VLAN broadcast domains, preventing Layer 2 ARP frames from reaching the default gateway.
  4. D
    The switch port was placed into a Spanning Tree Protocol (STP) blocking state because the database server transmitted BPDU frames with a lower bridge priority.

Cevap

The switch interface defaulted to half-duplex operation after failing auto-negotiation with the forced full-duplex server NIC.
When a network device interface is manually configured for full duplex without auto-negotiation enabled, the connected switch port set to auto-negotiation can detect the link speed (100 Mbps) via clock pulses, but fails to negotiate duplex parameters. Standard Ethernet behavior dictates that an auto-negotiating interface must default to half-duplex when duplex negotiation fails. Consequently, the server transmits whenever it has data (full duplex), while the switch expects half-duplex CSMA/CD rules. When the switch transmits while simultaneously receiving data from the server, the switch registers a collision after transmitting the first 64 bytes of the frame, logging a late collision.

Adım Adım Çözüm

1
Analyze the observed interface error counters.
Identified high counts of late collisions, FCS errors, and alignment errors on FastEthernet0/12.
Late collisions occur when a device transmits data after its collision window (first 64 bytes) has passed, which is a key indicator of a duplex mismatch.
2
Evaluate the configuration settings on both link endpoints.
The server NIC is forced to 100 Mbps Full Duplex, while the switch port relies on auto-negotiation.
IEEE 802.3 auto-negotiation requires both ends to participate. If one end is forced, the auto-negotiating end can sense speed (100 Mbps) but cannot determine duplex mode.
3
Apply Ethernet standard fallback rules for duplex negotiation.
The switch port sets its speed to 100 Mbps and defaults its duplex to half-duplex.
The default fallback for auto-negotiation when duplex cannot be negotiated is half-duplex. The server sends frames continuously (full-duplex), while the switch listens for line activity and detects collisions during its own transmissions, logging late collisions.

Anahtar Kavram

Ethernet Auto-Negotiation and Duplex Mismatch Symptoms
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Konular

Troubleshooting VLAN, Trunking, and Switching Issues

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