i am looking eagerly to know the potential impact on network while and after changing maximum transmissions (MTU ) from 1500 default value to any other value .

  • The 1500 MTU is only for ethernet. Other protocols have other MTU sizes. For example, the Wi-Fi MTU is 2304, token ring is 4500, serial communication is usually somewhere over 4000. Some ethernet devices can use jumbo frames, but that is non-standard, and what a device can handle can vary by vendor, device model, and even different interfaces on the same device. Unless you are very careful, you can break the network with jumbo frames.
    – Ron Maupin
    Feb 8 at 13:22

Increasing the MTU requires that the underlying link layer (e.g. Ethernet with a standard maximum payload of 1500 bytes) can actually handle the new maximum frame size. Since frame sizes are not negotiated you need to make sure that all connected devices in that segment can handle the new frame size. This may significantly increase the processing overhead on some devices (jumbo frames/packets may require processing in software/by the CPU that is otherwise handled by hardware/offloading).

Also, routing from large-to-small MTU areas can put significant stress on the IPv4 gateways due to required fragmentation.

Decreasing the MTU increases the relative header overhead but may be reasonable to avoid in-path fragmentation, esp. when you're tunneling - using the standard MTU may increase fragmentation through a tunnel (unless the tunnel endpoints can use larger-than-standard packets). Tunneling requires additional encapsulation, and the additional headers decrease the effective payload capacity of a packet/frame. Without decreasing the inital payload, that leads to massive fragmentation with IPv4.

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