On Wed, May 12, 2021 at 6:12 PM Ron W <ronw.m...@gmail.com> wrote: > I am trying to use WireShark to diagnose a network problem between a > Windows PC and a Linux-based controller (for a robot). > > The controller uses uboot and TFTP to download the Linux image from the > PC. Using the controller's serial port, I can see the messages output by > uboot and by Linux. The messages as as expected and the controller appears > to work correctly except after downloading Linus via TFTP, the PC > application is not able to communicate with the controller via TCP/IP. > > So, I connected an Ethernet switch between the PC and the controller and > also connected a laptop to the switch so I can monitor with WireShark. >
Your problem is the Ethernet switch you introduced to allow a 3rd device to (attempt to) listen in on the conversation. You can't do that using a switch. A switch routes messages from one port directly to the port belonging to the destination. It won't get sent to all the other ports (i.e. your sniffer). What you need is a hub, not a switch. Hubs send everything to all ports. Switches only send broadcast messages to all ports. That's why I kept my 10Mb hub. Just for these circumstances. B.T.W. There are some 'managed' hubs that allow you to designate a 'sniffer' port that will receive all messages to/from a designated 'other' port. That feature is only available on the more expensive switches.
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