The concept of MTU (Maximum Transmission Unit) is relevant to both Layer 2 (Data Link Layer) and Layer 3 (Network Layer) of the OSI model. However, when people generally ask about MTU, they are often referring to the Layer 3 MTU, specifically the IP MTU.
Layer 2 MTU (Ethernet MTU)
This refers to the maximum size of a frame that can be transmitted over a data link layer protocol, such as Ethernet. For standard Ethernet, this is typically 1500 bytes for the payload (data portion). The complete Ethernet frame, including headers and trailer, is larger (around 1518 bytes). Higher Layer 2 MTUs (like 9000 bytes or more) are sometimes configured and known as Jumbo Frames.
Layer 3 MTU (IP MTU)
This refers to the maximum size of an IP packet, including the IP header and the data it carries. The IP MTU is important because Layer 3 is responsible for routing packets across networks. An IP packet cannot be larger than the MTU of the network path it traverses. If it is, fragmentation may occur.
Key Differences and Relationship:
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The IP MTU (Layer 3) is often configured to be smaller than or equal to the Ethernet MTU (Layer 2) to avoid fragmentation.
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The IP MTU refers to the maximum size of the IP packet itself.
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The Ethernet MTU refers to the maximum size of the entire Ethernet frame, which carries the IP packet.
Therefore, the MTU can be associated with both Layer 2 and Layer 3. The context of the question determines which MTU is being referred to. Generally, the term "MTU" without further qualification often implies the Layer 3 (IP) MTU.
