Re: [lwip-users] Understanding MTU and Max ETH Frame size

[Dimax]

Hi,

Thanks for your answer. But I need to clarify it

If I set MTU=1500 am I guaranteed not to get Ethernet frames bigger then 1514 bytes?

I need to know it to make proper DMA buffers allocation.

As far as I understand MTU is exchanged between two ETH peers via management packets and each side should respect peers MTU?

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On Tue, May 28, 2013 at 9:12 AM, Dimax <address@hidden> wrote:

Hi,
I'm working on new lwIP driver for STM32F2x7. This CPU has build in MAC and external PHY. Data from MAC to CPU memory is transferred by dedicated DMA.
To define driver strategy I want to better understand what can be a Max Ethernet Frame size that driver can receive from MAC?
If I get it right I should not get packets longer then MTU ?
Am I right?
If yes why I do not see MTU configuration in lwipopts.h ? All I can see is
/* TCP Maximum segment size. */
#define TCP_MSS                 (1500 - 40)
Does it mean that MTU=1500 is hardcoded value?

No. MTU is a member of struct netif and must be set per netif in its init function. For standard ethernet, this *is* 1500 (so a complete ethernet frame including headers is 1514 bytes long at maximum). You might be able to send/receive longer frames in your local subnet, though.
Simon