Quiz: MTU

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3 questions

L1 (2 questions)

1. How do you detect an MTU blackhole?

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ping -M do -s 1472 (don't fragment). If it fails at 1473 but works at 1472, MTU is 1500. Reduce size to find the actual path MTU. Symptoms: SSH works but large HTTPS pages hang.

2. What is MTU and what happens when a packet exceeds it?

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MTU (Maximum Transmission Unit) is the largest frame a link can carry (typically 1500 bytes for Ethernet). If a packet exceeds MTU and DF (Don't Fragment) is set, the router drops it and sends ICMP Fragmentation Needed. If DF is not set, the packet is fragmented.

L2 (1 questions)

1. Large file transfers fail but small requests work. What network issue should you suspect?

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MTU/PMTUD (Path MTU Discovery) problem. Small packets fit in any MTU; large ones hit a link with smaller MTU. If ICMP is blocked by a firewall, the sender never learns to reduce packet size. Fix: allow ICMP type 3 code 4, or reduce MTU on the sender's interface.