TCP retransmission with Port mirroring

[bartj]

Hello,

I am using some SG200-08 Cisco switches on a local network with some custom hardware I am debugging. I have setup one of the switches to mirror rx/tx traffic so that I can sniff packets with wireshark on a PC that is not part of the network. However, I find that the traffic as captured by wireshark on the PC that is accepting the mirrored packets duplicates every packet as a "TCP Retransmission".

I have attached 2 files, one showing the duplicate packets (Wireshark_dups.PNG) and another taken with wireshark on the host PC which is part of the network(Wireshark_no_dups.PNG). The later file contains no duplicate packets. Note that the 2 files are not time correlated, they serve only as examples of the packet duplication issue.

I have tried disabling spanning tree options but that makes no difference.

Does anyone have any suggestions as to how to get rid of: "TCP Retransmission" packets?

Thanks,

bartj

[dmuinoorallo]

Hello,

There are different causes for this, typical issues are disconnect ethernet cable or congestion in this interface.

I need more information about your shema of network for help you

[bartj]

Hello,

I have attached a PNG showing the current network configuration. I have changed it a bit from the last post so that I only see traffic to/from the HOST PC and the custom hardware. TCP re-transmission is still an issue. Any insights appreciated.

Thanks,

bartj

[dmuinoorallo]

In a TCP/IP Client-Server Model arch, TCP retransmission can happen ONLY when the transmitting end does not recieve TCP-ACK from the receiving end.

 Few possibilites of NOT receving TCP-ACK are,

• The receiving end sent back TCP-ACK is LOST in transit.
• The transmitting end TCP-DATA is LOST and it did not reach the receving end at all.

Check for error along the traffic path, problems in layer 1 and layer 2.

[bartj]

dmuinoorallo,

Thank you for your comments. Its interesting that there are TCP re-transmissions for both directions of traffic (HOST > Custom HW and Custom HW to HOST).

I will attempt to do more testing on Tuesday when I have access to the hardware.

Thanks again!

[dan.agache]

Hello,

We have the same problem with retransmission. How did you solve the problem?
Did you figure out the cause of retransmission?

Thank you for your help,
Dan