linux - MTU affect on bandwidth/throughput

Question

My ISP has capped my connection to 100 Mbps and I am connected to them on Fiber. This connection is being terminated in a Cisco router which is then connected to a Mikrotik device. I also have a Pi4 behing Mikrotik for load balancing. Everything works fine and my connection goes 100 Mbps Up/Down. But when I change the MTU from standard 1500 to anything larger, 2026 on Pi4 and Mikrotik, for example I get up/down speeds of 500 Mbps. Switch back to 1500 all the way back things come back to 100 Mbps instant.

LAN (1500) -> Pi4 (2026) -> Mikrotik (2026) -> Cisco (1500) -> ISP ..

I know this does not makes any sense but is it possible that MTU change can someone affect the way ISP is capping the port? Fragmentation, more packets ... anything?

Thanks.

question from:https://stackoverflow.com/questions/65879906/mtu-affect-on-bandwidth-throughput

Answer 1

The use of large MTU sizes allows the operating system to send fewer packets of a larger size to reach the same network throughput. The larger packets greatly reduce the processing required in the operating system, assuming the workload allows large messages to be sent.

Larger MTU sizes typically provide better overall performance than smaller MTU sizes. Larger MTU sizes reduce the number of packets needed to transfer a large amount of data.

ISP fiber connection traffic shaping does not depends on users MTU. ISP fiber connection settings will not allow you to use more than 100 Mbps unless it is configured incorrectly.