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Networking Fundamentals · Foundational

Maximum Transmission Unit (MTU)

The largest Layer 3 packet, in bytes, that a link can carry without fragmentation; 1500 bytes on standard Ethernet.

Definition

MTU is set per interface and determines how large an IP packet can be before it must be fragmented (IPv4) or dropped with an ICMP message (IPv6 or IPv4 with DF set). Tunnels and encapsulations such as GRE, IPsec, VXLAN and PPPoE add overhead and effectively reduce the usable MTU. Jumbo frames raise MTU to around 9000 bytes on supporting networks.

Example

PPPoE adds 8 bytes of overhead, so DSL links commonly use an MTU of 1492.

Exam tip

An MTU mismatch often shows as small pings succeeding while large transfers or web pages hang.

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