MTU, fragmentation and VPN problems from the client

Skills you'll practise

• Calculate ping payload sizes for a given MTU and find the path MTU with ping -f -l
• Recognize MTU black-hole symptoms
• Diagnose "VPN connected but apps don't work" using DNS, routes and MTU evidence

Course outline

1. 1.MTU, fragmentation and VPN problems from the clientVideo · 6 min
2. 2.Finding the path MTULesson · 14 min
3. 3.VPN connected, apps brokenLesson · 12 min
4. 4.Why tunnels shrink packets, and who fixes MTU problemsLesson · 12 min
5. 5.MTU, fragmentation and VPN problems from the client: knowledge checkKnowledge check · 20 questions
6. 6.MTU, fragmentation and VPN problems from the client: practical exerciseKnowledge check · 1 question
7. 7.VPN connected, internal apps failScenario
8. 8.Final exam5 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.

• RFC 1191: Path MTU Discovery
• RFC 4821: Packetization Layer Path MTU Discovery
• Microsoft Learn: ping command reference
• Microsoft Learn: netsh interface ipv4 show subinterfaces