If tcpdump on the originating system tells you a frame has been sent, then unless the NIC is defective, it has made it onto the physical network.
If tcpdump on the receiving end tells to a frame was never received, then unless the NIC is defective, it never arrived (or arrived with a bad frame check sequence).
You should investigate the possibility of faulty hardware. Check the link and advertised capabilities with ethtool or mii-tool, check the statistics at both nodes with ifconfig, take a look at the logs, see if [url="iperf.sourceforge.net"]iperf or a flood ping results in dropped packets/frames, check cabling, see if the switch reports any errors (and try bypassing it altogether) etc.
Edit: It is also possible that a bug has been introduced in the driver for your NIC in later kernels. Also, the driver code may have been updated, exposing a previously unnoticed hardware issue.
Last edited by Ser Olmy; 05-26-2012 at 11:34 AM.
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