I will first describe my experiment environment in details.
There are two DELL optiplex 990 computers and each one has two PCI-E x16 slots in blue and black. I plug in two Intel PRO/1000 PT Quard Port network cards in each PC. So totally 4 slots and 4 network cards.
The problem is when I use Iperf as a traffic measurement tool to test the throughput between two PCs in 100M speed mode, the network card plug in the black slot worked fine, but the network card plug in the blue one can't be used for Iperf, they can only be used as ping, traceroute, etc. The computer will die when I used Iperf in blue slots network card.
I have checked all four network cards, they are fine. So the problem is in the blue slot. I have no idea whether the blue slot can only work for the video card? But the computer has a integrated video card and works fine.
