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I have a HP Procurve switch and have configured port 1 on the switch as a span / mirror port.

I also have a workstation with 3 interface running Ubuntu 10.04. When I connect eth0 which I have already set to promisc mode to port 1 of the switch, it does not connect.

However, when I connect eth0 any other port on the switch (e.g. 2, 3) it will connect and automatically grab an IP address from DHCP.

Any idea what went wrong?

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Hello! Can you please try to set a fixed address to your eth0, connect to port 1 and use Wireshark or ethereal to see if you get any DHCP packets? – Paulo Arruda Nov 23 '11 at 14:34

The "span" mode on a switch is intended to let you connect a standard cable from that port to a normal port on another switch by switching the transmit and receive lines (i.e. so data transmitted by one switch hits the receive lines on the other). This is generally equivalent to using a cross-over cable to connect two normal ports on two switches. It doesn't have anything to do with enabling promiscuous mode on a network card

The ports on network interface cards do not expect the transmit/receive wires to be crossed this way, so when the switch tries to send data to your computer, it is arriving on the transmit lines rather than the receive lines and vice versa.

There are some network cards that would work with the setup you've described, but they would just be detecting the cross over and correcting for it, rather than providing any more features over a standard card.

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The description above matches mdix, the detection of whether a straight or crossover cable is connected to a specific port. This has nothing to do with the problem described by Frank, nor with configuring span ports. – user164089 Nov 13 '15 at 10:10

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