What does the Layer 2 MTU include that Layer 3 MTU does not?

The Layer 2 Maximum Transmission Unit (MTU) refers to the largest size packet that can be transmitted over a particular Layer 2 network. It includes all the data from the Ethernet frame, which consists of the Ethernet header and the Layer 3 packet (such as an IP packet).

In contrast, the Layer 3 MTU specifically pertains to the size of the payload that can be transported by Layer 3 protocols, such as IP. This does not encompass the Layer 2 header, which is critical for the delivery of frames in an Ethernet network.

Thus, the Layer 2 MTU calculation incorporates the Ethernet header, which is essential for the framing of the data at Layer 2, including addressing and error-checking information necessary for the transmission over the physical medium. The Layer 3 MTU disregards this header since it only focuses on the encapsulated data (the Layer 3 packet and its headers), leading to a distinction where the Ethernet header is part of the Layer 2 MTU but not the Layer 3 MTU.