Cisco Certified Support Technician (CCST) NetworkingNetwork AccessHard
A network professional is troubleshooting a slow network connection for a user. The user's computer is connected to a 100 Mbps switch port, but the 'show interface' command on the switch reveals 'Half Duplex' for that port, while the computer's network adapter is configured for 'Auto-negotiate'. This mismatch is causing significant performance degradation. What is the MOST common symptom of a duplex mismatch?
- AIncreased latency and frequent disconnections.
- BComplete loss of network connectivity.
- CExcessive collisions and retransmissions, leading to very slow performance.
- DFailure to obtain an IP address via DHCP.
Show answer & explanationAnswer & explanation
Correct answer: C. Excessive collisions and retransmissions, leading to very slow performance.
A duplex mismatch occurs when one side of an Ethernet link operates in full-duplex mode and the other operates in half-duplex. When the full-duplex side transmits, the half-duplex side may detect this as a collision, leading to excessive collisions, frame errors, and retransmissions, severely degrading performance to the point of being unusable.
Why the other options are wrong
- A. Increased latency is a symptom, but 'frequent disconnections' is less common than severe performance degradation due to collisions.
- B. While performance is severely impacted, a duplex mismatch typically does not cause a complete loss of connectivity; some traffic may still pass, albeit very slowly.
- D. Failure to obtain an IP address is usually due to DHCP server issues, incorrect VLANs, or a physical link problem, not specifically a duplex mismatch (though the slow connection might indirectly hamper DHCP).
Duplex Mismatch
A duplex mismatch occurs when two connected Ethernet devices have different duplex settings (one full-duplex, one half-duplex), leading to severe network performance issues.
- Causes excessive collisions and retransmissions.
- Results in extremely slow network performance.
- Often happens when one side is set to auto-negotiate and the other is hard-coded to half-duplex.
- Always configure both sides to auto-negotiate or hard-code both sides identically.
Memory trick: Half-Full Fight, Collisions Take Flight