Thank you, that is interesting. Doesn't it also imply is that with the smaller MTU of 590 you'll end up having fragmentation which will add a little bit of processing overhead? I understand it is not that big of a deal since fragmentation happens a lot, but for the best performance yes, both sides have to agree on a decent MTU.
On Thu, Mar 4, 2021 at 2:39 PM Gregory Nutt <spudan...@gmail.com> wrote: > > John, to answer your question on ethernet, 1500 is a very common MTU. For > > VLAN tagged frame support (802.1Q VLAN ID ), 1518 bytes (1522 bytes on > the > > wire with 4-byte / 32-bit ETH CRC32), for Jumbo frames (not IEEE) 9122 is > > common. Minimum frame size with CRC is 64-bytes and zero padding is > common, > > < 64 is considered a runt frame. And note if ethernet_len field is 0x800 > > you have an IP packet. Good luck on STM32H7 ethernet hardfault debug. > > The MTU is a property of a network that is controlled by the network > configuration. These are the "standard" MTUs used on commercial > networks and are pervasive in commercial networks. Other MTUs are > theoretically possible for custom networks but are not supported by the > H7 Ethernet MAC. > > The value 590 is the minimum size that you can use for a fully > functional network. It is not commonly used but perfectly valid. > Increasing this default would break some configurations. Some older > parts, such as LPC17xx have plenty of SRAM, but are constrained to a > very small bank of DMA-able memory for Ethernet and USB. > > Ethernet MACs that are not able to support certain packet sizes need to > have checks to prohibit selection of sizes that they cannot support. > > >
