MTU (Maximum Transmission Unit) is the largest packet a link can carry.
Common values:
- Ethernet: bytes
- PPPoE: bytes
- VPN tunnels: or less
- Jumbo frames: up to bytes
MTU includes the IP header but not Ethernet header. When packets exceed MTU, they get fragmented or dropped.
MTU mismatches cause mysterious problems. Packets work until hitting a low-MTU link, then connections stall.