Re: config problem - not seeing all messages

[Fulko Hew <fulko.hew () gmail com>]

On Wed, May 12, 2021 at 6:12 PM Ron W <ronw.mrmx () 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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