What Is MTU?
MTU — the Maximum Transmission Unit — the largest packet size (in bytes) an interface will send without fragmenting. Ethernet's default is 1500 bytes.
How it works
Packets bigger than a link's MTU are either fragmented (split, with reassembly at the destination) or dropped — dropped with an ICMP "too big" message if the packet set the Don't Fragment flag. Tunnels (VPN, GRE) add headers, effectively shrinking usable MTU and causing classic weird-breakage symptoms.
Why it matters
MTU mismatches produce infamous faults: pings work but web pages hang, small transfers fine, big ones stall. Engineers test with ping using size and DF options. Jumbo frames (9000 bytes) raise MTU inside data centres for efficiency.
Frequently asked questions
What is the default MTU?
1500 bytes on standard Ethernet interfaces.
What happens if a packet exceeds the MTU?
It's fragmented into smaller pieces, or dropped (with an ICMP notification) if the Don't Fragment flag is set.
Why do VPNs cause MTU problems?
Tunnel headers consume part of the 1500 bytes, so full-size inner packets no longer fit — causing drops or fragmentation until MTU/MSS is adjusted.
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