VPN Policy & MTU Troubleshooting
Routing
Overview
Encapsulation reduces space for an inner packet. If the underlay MTU is 1500 and modeled tunnel overhead is 80 bytes, inner MTU is 1420. Path MTU discovery and ICMP messages prevent black holes; filtering those messages can break large transfers while small pings work.
Protocol stages
- 1Underlay MTU
- 2Tunnel overhead
- 3Inner MTU
- 4Path MTU evidence
Interactive exercise
Knowledge check
Underlay MTU 1500, overhead 80, inner packet 1450 with no fragmentation. Will it fit?
Educational model — not real hardware, traffic or official certification.