Hi Michael,
That’s right, there are two MTUs…the L2 and L3 MTU. On a switch that supports Jumbo frames, there’s even a third MTU for that.
The L2 MTU doesn’t include the Ethernet frame header/trailer, it only specifies the size of the payload. This is probably why it is confusing since both MTUs are 1500 bytes by default.
The main difference is that the L2 MTU specifies what the interface is able to send, the L3 MTU specifies when we fragment the packet.
Rene