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


From: Sylvain Rochet
Subject: Re: [lwip-users] Understanding MTU and Max ETH Frame size
Date: Wed, 29 May 2013 09:59:39 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

Hi Dimax,


On Tue, May 28, 2013 at 03:19:41PM +0300, Dimax wrote:
> 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?

You are not, MTU is Maximum -Transmission- Unit, not receive. You might 
receive giant frames from your Ethernet scope (802.1q, 802.1ad, MPLS 
Frames, ...), but your hardware MAC should drop giant frames if you 
haven't configured it to handle/pass them.


> 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?

MTU is not exchanged, at least not on the Ethernet layer. All hosts on a 
Ethernet subnet should have the same configured MTU. On the IP side, 
ICMP might be used to discover the maximum usable MTU over a layer 3 
network path.


Sylvain

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