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.
