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Difficulty: MediumWired and Wireless Network Issues

A network technician is troubleshooting an IP phone in a newly configured corporate office. The phone boots successfully but experiences heavy packet loss, distorted audio, and intermittent network disconnects during active calls. The technician verifies that the switch port connected to the phone is manually locked to 1 Gbps Full Duplex, whereas the phone's network interface port is set to Auto-Negotiate and running at 100 Mbps Half Duplex. Which of the following is the MOST likely cause of the performance degradation?

  1. A speed and duplex mismatch between the switch port and the phone interface causing excessive collisions and packet drops.Answer
  2. B
    The phone was automatically assigned an APIPA address due to an unreachable DHCP server on the voice VLAN.
  3. C
    The local DNS server is incorrectly attempting to assign IP address leases to client devices.
  4. D
    The Ethernet patch cable has a severed pin connection that turned off the switch interface link light.

Answer

A speed and duplex mismatch between the switch port and the phone interface causing excessive collisions and packet drops.
When a network switch port is manually locked to Full Duplex while the connected endpoint is set to Auto-Negotiate, the endpoint cannot detect the duplex setting and defaults to Half Duplex. This duplex mismatch causes the full-duplex end to send data continuously while the half-duplex end senses collisions, generating late collisions, frame check sequence (FCS) errors, and severe packet loss on real-time traffic such as VoIP.

Step-by-Step Solution

1
Analyze the reported symptoms and diagnostic findings.
The IP phone boots and connects to the network but experiences poor quality, packet loss, and drops. The switch port is locked to Full Duplex while the phone runs at Half Duplex.
Identifying mismatched interface configurations explains why physical connectivity exists but performance is severely impacted.
2
Evaluate the behavior of Ethernet auto-negotiation.
When one end of a link is manually fixed to Full Duplex without auto-negotiation, the auto-negotiating side fails to detect the duplex mode and defaults to Half Duplex per standard IEEE rules.
Auto-negotiation requires both endpoints to participate; otherwise, fallback rules apply.
3
Determine the impact of a duplex mismatch on data transmission.
The full-duplex switch port transmits data without checking for carrier activity, while the half-duplex phone port interprets concurrent transmissions as collisions, leading to late collisions and dropped packets.
Duplex mismatches cause severe frame loss, particularly under real-time streaming traffic like VoIP.

Key Concept

Wired Network Troubleshooting: Speed and Duplex Mismatch
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Topics

Wired and Wireless Network Issues

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