Link speed, duplex and MTU

Skills you'll practise

• Read link speed, duplex and error counters on an interface
• Explain the symptoms of a duplex mismatch
• Test path MTU with a do-not-fragment ping and calculate payload sizes
• Explain why a gigabit link that negotiates at 100 Mb/s points to a cabling problem

Course outline

1. 1.Link speed, duplex and MTU problemsVideo · 2 min
2. 2.Negotiation, duplex and errorsLesson · 15 min
3. 3.MTU and fragmentationLesson · 15 min
4. 4.Reading interface counters and fixing errorsLesson · 15 min
5. 5.Link speed, duplex and MTU: knowledge checkKnowledge check · 19 questions
6. 6.The file server is slow only for big filesScenario
7. 7.Final exam7 questions · passing it completes the course, so people who already know the material can test out

Sources it draws on

The lessons and questions are written from these references, so learners can go back to the original.

• IEEE 802.3 Ethernet (auto-negotiation, 1000BASE-T)
• RFC 1191: Path MTU Discovery
• RFC 8201: Path MTU Discovery for IP version 6
• WireGuard default MTU of 1420 bytes (wg-quick)