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Subject: Re: Ethernet question
From: Heinz Junkes via Tech-talk <tech-talk@aps.anl.gov>
To: Mark Rivers <rivers@cars.uchicago.edu>
Cc: "tech-talk@aps.anl.gov" <tech-talk@aps.anl.gov>
Date: Tue, 24 Dec 2019 12:48:37 +0100
Hello, Mark,

I thought also about the MTU right away, just like Michael. With 10Gbit/s Ethernet this is 9000 bytes 
as default and not 1500 bytes as with the 1Gbit/s Ethernet.
We had similar problems with a small Omron PLC. It only supported 10Mbit/s and it could handle
UDP even only with a MTU of 768 bytes. Omron's internal buffer was simply designed too small. 
Here we had to connect a router to make it work.

Heinz  

P.S.

I found this in 82599-datasheet-v3-4.pdf:



On 23. Dec 2019, at 23:46, Mark Rivers via Tech-talk <tech-talk@aps.anl.gov> wrote:

Folks,
 
I am having a strange problem which I am wondering if someone might shed some light on.
 
I have some old Ethernet data acquisition devices, the Canberra AIM MCAs.  These are not TCP/IP, but rather use the IEEE 802.2 Extended SNAP protocol.  I have been talking to these devices for many years from vxWorks, Windows (using WinPCAP) and Linux (using libnet and libpcap).  I recently found that I was not able to talk to them from a number of my Linux systems.  It partly works: I can build a list of the devices on the network, I can take “ownership” of the device, and I can read what auxiliary Instrument Control Bus devices are connected to it (ADCs, amplifiers, HVPS, etc.).  However, some messages I send to the device never get answered.  When I look with Wireshark and compare the packet sent with a Windows machine and the non-functioning Linux machine the packets look the same.  But the Windows machine gets a reply and Linux does not.
 
On further investigation I found that the problem only occurs when using 10 Gbit Ethernet adapters.  When using a 1 Gbit adapter it works fine.  I was able to see this in a single machine that has both 10 Gbit and 1 Gbit adapters.  If I use the 1 Gbit adapter it works fine, if I use the 10 Gbit adapter it fails (but kind of works as described above).  On 3 separate systems 1 Gbit works, and 3 other systems 10 Gbit fails.
 
This does not make sense to me.  The Canberra device itself is 10 Mbit Ethernet.  It is plugged into a 1 Gbit switch.  The packets are all “slowed-down” in the switch to 10 Mbit, so I don’t understand why it matters if the network card is 1 Gbit or 10 Gbit.
 
I also used ethtool to lower the speed/duplex of the 10000T interface to 1000/Full and then to 100/Full and that did not help.  I could still see the module, but much functionality does not work.  So a 1 Gbit adapter works, but a 10 Gbit adapter slowed down to 1 Gbit or even 100 Mbit does not work.
 
Any ideas what can cause these symptoms?
 
Thanks,
Mark


References:
Ethernet question Mark Rivers via Tech-talk

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