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Maximum Transmission Unit (MTU)

Maximum Transmission Unit (MTU) defines the largest frame size allowed on a network, with standard Ethernet capped at 1,522 bytes total and a usable payload of 1,500 bytes. This limit prevents devices from overwhelming network hardware by sending arbitrarily large frames.

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About this video

Maximum Transmission Unit, commonly referred to as MTU, is a network parameter that defines the largest frame size a device is permitted to transmit. This limit exists because, without it, a sending device could generate frames large enough to overwhelm the processing hardware on the receiving end, creating a potential denial-of-service condition at the hardware level. Establishing a maximum frame size ensures that all devices on a network operate within predictable, manageable boundaries. For standard Ethernet at Layer 2, the MTU is set to 1,522 bytes. Of those bytes, 22 are consumed by the frame header and trailer, leaving a maximum usable payload of 1,500 bytes. It is this payload figure, 1,500 bytes, that is most relevant in practice, as it represents the actual data capacity of each frame. Although the MTU can technically be modified, any change must be applied consistently across every piece of equipment in the network path. Because of this operational complexity, the vast majority of networks continue to use the 1,522-byte Ethernet standard without modification.

What you'll learn

What's covered

Maximum Transmission Unit (MTU)

Aligned to

CompTIA Network+
1.1 Explain concepts related to the Open Systems Interconnection (OSI) reference model.
5.4 Given a scenario, troubleshoot common performance issues.
Cisco CCNA
1.3 Compare physical interface and cabling types
Cisco CCST Networking
1.1 Identify the fundamental conceptual building blocks of networks

Key terms

Maximum Transmission Unit
MTU
The largest size of a packet or frame that can be transmitted over a network link without fragmentation, typically 1500 bytes for Ethernet. When packets exceed the MTU, they must be fragmented at Layer 3 or the sender must reduce the packet size via Path MTU Discovery.
Data Link Layer
Layer 2 of the OSI model responsible for node-to-node data transfer and error detection on a single network segment.
Payload
The actual data content of a packet or message, excluding headers and metadata.
Encapsulation
The process of wrapping data with protocol headers as it passes down the layers of the OSI model.

Transcript

Why we limit frame size

One of the problems is that if a device were to just be allowed to send any size of frame that it wanted to, it could capitalize on the hardware on the other side and overwhelm the hardware on the other side. So we have to set some sort of limits. That's what an MTU is for.

MTU is the maximum transmission unit. Within a layer 2 Ethernet frame, the maximum transmission unit by default is 1,522 octets, and this allows for a payload total of 1,500. Of course, we have the 22 extra octets because of what comes in the header and trailer there.

So that is the maximum transmission unit: how big a frame can be as it gets sent out. Once again, the standard is 1522, although there are some abilities to change that, although you have to change it on all of your equipment. So most of the time, most of our networks, we just use the standard. That's the maximum transmission unit, the max amount that we can actually transmit.

One thing to note is that even though we say the maximum transmission unit for an Ethernet frame is 1,522 bytes, really it has to deal with the payload, and the payload itself has a maximum transmission unit of 1500.

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