5.4 Diagnosing Common Enterprise Failures: Duplex Mismatches, MTU Black Holes & Rogue Services
Examines real-world fault resolution for duplex mismatches, MTU black holes, rogue DHCP servers, and optical transceiver wavelength mismatches.
🎯 Key CompTIA Network+ Exam Takeaways
- Late collisions and CRC alignment errors are the textbook indicators of an Ethernet speed/duplex mismatch.
- MTU black holes cause small ping packets to succeed while large web/VPN data payloads freeze because ICMP errors are dropped.
- Optical transceivers must match in wavelength and media type: 10GBASE-LR (1310nm single-mode) cannot connect to 10GBASE-SR (850nm multimode).
Duplex mismatches occur when one end of an Ethernet link is set to Full-Duplex while the other operates in Half-Duplex. The full-duplex end transmits without checking the carrier, causing late collisions and CRC alignment errors on the half-duplex end.
Maximum Transmission Unit (MTU) mismatches occur when IPsec VPN encapsulation reduces available MTU. If client packets have the Don't Fragment (DF) bit set and intermediary routers block ICMP 'Fragmentation Needed' replies, large HTTPS transfers hang in an MTU Black Hole.
When troubleshooting fiber links, engineers must verify that optical transceivers match in wavelength and fiber core type. A 1310 nm long-reach transceiver will not establish link light with an 850 nm short-reach optic.
⚠️ Common Pearson VUE Exam Traps
- Assuming an MTU black hole is a total loss of connectivity; ping and small packets still succeed normally.
- Overlooking rogue DHCP servers when workstations unexpectedly receive 192.168.1.x addresses on corporate 10.x networks.
Knowledge Checkpoint
A workstation connected to an Ethernet switch port experiences extremely slow network file transfers. When inspecting the switch port statistics, the technician notices hundreds of 'Late Collisions' and CRC alignment errors, while the workstation NIC reports zero collisions. What is the root cause of this issue?