MTU and Jumbo Frames

The maximum transmission unit (MTU) is the largest payload a network interface can send in a single frame without fragmenting it. On Ethernet, the standard MTU is 1500 bytes. A standard Ethernet frame adds 14 bytes of header and 4 bytes of CRC on top of that payload, for a total frame size of 1518 bytes.

Most network equipment defaults to the standard 1500-byte MTU, and this is the right choice for general-purpose networks, including anything that touches the public internet.

A jumbo frame is any Ethernet frame with a payload larger than 1500 bytes. They are not part of the IEEE 802.3 standard, so support and maximum size vary by vendor. In practice, 9000 bytes is the de facto ceiling most networking equipment supports, and it’s the value most commonly meant by “jumbo frames.” Frames above 9000 bytes are sometimes called super jumbo frames, up to a theoretical maximum of 65535 bytes; these are rarely used outside specialized IPv6 jumbogram scenarios.

Jumbo frames reduce per-packet overhead. Moving the same amount of data in fewer, larger frames means fewer headers to generate, transmit, and process, which lowers both bandwidth spent on overhead and CPU cycles spent handling packets. The gains are only meaningful on networks that are already fast (1 Gbps or higher) and under real load; on a lightly used or slower network the difference is negligible.

The tradeoff is fragility. Every device in the path (switches, routers, NICs, virtual network interfaces) needs to agree on the larger frame size. If one hop doesn’t, the result is either fragmentation (IPv4) or a dropped packet (IPv6, which does not fragment). Because jumbo frames must be enabled manually and aren’t uniform by default, a single misconfigured device on the path can cause hard-to-diagnose throughput problems rather than an outright failure.

For that reason, jumbo frames are best reserved for controlled, single-purpose network segments where every device is known and configured consistently. Storage area networks and high-performance computing clusters are the classic use case. They should not be enabled on general office or home networks, and never on interfaces facing the public internet.

The TCP maximum segment size (MSS) is a related but separate value: it’s the largest chunk of data a TCP endpoint will accept in a single segment, negotiated by both sides during the TCP handshake. Unlike MTU, MSS refers only to the TCP payload and excludes the TCP and IP headers. MSS is normally derived from the interface MTU (MTU minus IP and TCP header overhead), so when jumbo frames are enabled end to end, MSS increases correspondingly and each TCP segment carries more data per round trip.

Jumbo frames can meaningfully speed up large data transfers between sources, stores and destinations when the entire path between them, physical and virtual, is under your control and consistently configured. They are not something to enable by default, and are not appropriate on any segment that touches an uncontrolled network or the internet.

By default, the Plakar Control Plane appliance uses the standard 1500-byte MTU on its interfaces. This is configurable per interface from the settings page if your network requires a different value.