I am attempting to get an embedded board working that using TCP/IP and UDP/IP. I have it working so that I can issue HTTP POST and GET commands and UDP works fine but am struggling with SNTP, so I have bought a managed switch so I can mirror what my embedded board is sending out and try to track NTP packets in and out.

I have the following set-up:

Embedded board Desktop running Apache server on Linux: (Also sends receives custom UDP packets).

Using a normal switch the embedded board can GET and POST with the Apache server just fine and Wireshark, running on can capture the packets.

However, if I connect up the Linux machine and the embedded board to the managed switch, the GET and POST stops working. The ACK of the SYN,SYN-ACK,ACK doesn't work properly and the whole system fails to POST and GET correctly.

I have the embedded board connected to port 2 and the desktop to port 5. I have selected, on the switch management web page that the mirroring port is port 5 and mirrored port 2, with port 2 ingress and egress enabled. I've tried that the other way round too, so that mirroring port is port 2 and mirrored is port 5.

This is the first time I've ever used a managed switch so I'm probably doing something incredibly naive or stupid. Is this enough information to help advise me what I might be doing wrong? If not, I will supply any info that would help.

Many thanks!

Here's a JPEG of the set-up using the web interface for the switch

  • "managed switch" is a very broad term. Please tell us the brand, type and show relevant configuration.
    – Teun Vink
    Dec 16, 2014 at 16:14
  • It's TP-Link TL-SG3424. Are you looking for a screenshot of how I've set it up using the web interface?
    – DiBosco
    Dec 16, 2014 at 16:15
  • If so, I've added that now.
    – DiBosco
    Dec 16, 2014 at 16:20
  • I'm confused. The desktop is running the Apache server? So there's only two devices- the board and the desktop?
    – Ron Trunk
    Dec 16, 2014 at 16:22
  • Yes, Ron. Only two devices. Desktop which is my main machine is also running an Apache server. This is the machine running Wireshark too. Is that the issue? Do I need a third machine to do all the sniffing and leave the desktop and embedded board to work as normal?
    – DiBosco
    Dec 16, 2014 at 16:29

1 Answer 1



Yes you need a third machine. The port that receives the mirrored traffic can't receive regular traffic. That's why things aren't working. If you have a second interface on your desktop, you could possibly use that and bind Wireshark to that interface.

  • No, it can't, because when the embedded board sends out NTP requests it goes nowhere near the desktop. This is when I need the managed switch so I can sniff packets that are not bound for the desktop.
    – DiBosco
    Dec 16, 2014 at 16:36
  • I see. The ntp destination is a different address? What device is that?
    – Ron Trunk
    Dec 16, 2014 at 16:37
  • The NTP device is a server out on the Internet. For the now I have just chosen an IP address that my desktop queries when it is getting its NTP timestamps.
    – DiBosco
    Dec 16, 2014 at 16:40
  • I changed my answer to reflect this new information.
    – Ron Trunk
    Dec 16, 2014 at 16:45
  • Smashing, thanks. I'll dig out my laptop and get Wireshark going on that then.
    – DiBosco
    Dec 16, 2014 at 16:49

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