Manual:Maximum Transmission Unit on RouterBoards
Originally MTU was introduced because of the high error rates and low speed of communications. Fragmentation of the data stream gives ability to correct corruption errors only by resending corrupted fragment, not the whole stream. Also on low speed connections such as modems it can take too much time to send a big fragment, so in this case communication is possible only with smaller fragments.
But in present days we have much lower error rates and higher speed of communication, this opens a possibility to increase the value of MTU. By increasing value of MTU we will result in less protocol overhead and reduce CPU utilization mostly due to interrupt reduction.
There are several different values of MTU out there causing some confusion:
1) 1518 bytes - for standard Ethernet frame this include Layer 2 header (14 bytes) and FCS (Frame Check Sum) 4bytes - this is MTU of the frames that travel thought the wires 2) 1514 bytes - for standard Ethernet frame this include Layer 2 header (14 bytes), but without FCS - This is MTU of the frame that software work with 3) 1500 bytes - for standard Layer 3 (IP) packets (standard Ethernet frame without Layer 2 header and without FCS - This is value that can be configured in MikroTik RouterOS
So basically all those values are the exactly the same thing only taken in different places