# Question about low level ETH init

**URL:** <https://forums.freertos.org/t/question-about-low-level-eth-init/13139>\
**Category:** ST\
**Created:** [July 30, 2021, 8:19am UTC](https://forums.freertos.org/t/question-about-low-level-eth-init/13139 "2021-07-30T08:19:05Z")\
**Posts on this page:** 1\
**Showing post:** 2

<div class="post-metadata">

**Author:** ![hs2](https://avatars.discourse-cdn.com/v4/letter/h/a4c791/32.png) [@hs2](https://forums.freertos.org/u/hs2)\
**Post date:** [July 30, 2021, 5:09pm UTC](https://forums.freertos.org/t/question-about-low-level-eth-init/13139/2 "2021-07-30T17:09:10Z")

</div>

The peripheral should be set to a DMA buffer size covering the packet and matching the peripheral (DMA) requirements. For STM32F4 I found 1536 is a good size (as far as I remember) supporting std. MTU size of 1500.

Edit: Found one of the helpful posts from @htibosch

> [@How to define the frame size in FreeRTOS+TCP?](https://forums.freertos.org/t/how-to-define-the-frame-size-in-freertos-tcp/7877):
>
> [edeviser](https://sourceforge.net/u/edeviser/) wrote on [Friday, May 24, 2019](https://sourceforge.net/p/freertos/discussion/382005/thread/60e9ef1774/#9d3e): I am running FreeRTOS+TCP and I would like to decrease the MTU. Inside the stm32f4x7\_eth\_conf.h i find this: #define ETH\_RX\_BUF\_SIZE ETH\_MAX\_PACKET\_SIZE /\* buffer size for receive \*/ ETH\_MAX\_PACKET\_SIZE is defined in stm32f4x7\_eth.h to 1536. Further more in FreeRTOSIPConfig.h i found this: #define ipconfigNETWORK\_MTU 1500 What is the difference between ETH\_RX\_BUF\_SIZE and ipconfigNETWORK\_MTU? Where should I reduce the frame size?

Related to your question.

---

_[View the full topic](https://forums.freertos.org/t/question-about-low-level-eth-init/13139)._