Retransmissions in the LAN? Anomaly for sure, right?

Unanswered Question
May 12th, 2008

Guys, this in the LAN:


I already checked switch port duplex and speed ports. Server NIC is set to '1000Mbps auto'. All connections are gig.

I perform an application test (9MB of data transferred) and I see 700 packets retranmistted, out of order packets and checksum errors (I took a network trace).

Do you agree we have anomalies here? I mean, in the LAN, I should not see retransmissions like this, correct?

I have this problem too.
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gpulos Mon, 05/12/2008 - 12:42

In a good LAN you will not see retransmissions.....for the most part.

Some may be normal depending on your LAN type, bandwidth available, etc.

Alot are a problem that should be resolved. Usually a retransmission is due to network congestion problems; as well as other issues as you may know can be caused by quite a few things from cabling to bandwidth consumption to TCP stack to queueing problems....bottom line, a segment was dropped from the network and TCP had to either restart the session with a SYN or if the hosts supported 'fast retransmission' they could have used that before the tcp retransmission timer expired.

In your case, the fast retransmission doesn't look like it worked which may point to heavy congestion or a cabling problem if not a NIC/driver issue.

Use the 'show tcp' command on IOS to view tcp connectivity while the ftp is in session.

Please see the following links for troubleshooting Retransmissions:

rfc 2988:

cisco tcp:

Richard Burts Mon, 05/12/2008 - 13:04


I wonder if there is a clue about the issue in your post when you say: "Server NIC is set to '1000Mbps auto" Frequently when you set the speed the device will not negotiate the duplex and will default to half duplex. I wonder if there is a duplex mismatch that is causing drops and retransmissions. Can you set the server for auto auto, or can you set it for 1000Mbps full, or can you set the switch for half duplex and see if any of these changes reduce the amount of retransmission?




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