How to use a switch as a network tap?

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http://serverfault.com – I would like to tcpdump all traffic that my router does when it makes a firmware update. So I have taken a HP ProCurve 1800-8G switch and mirrored port 7 to port 8. I have connected: Internet connection in port 6 routers WAN port in port 7 Linux host running tcpdump in port 8 I suppose the router have a dhcp client on the WAN interface. However I don't see any activity. Not even the switch leds show activity for port 6 and 7. Question Do I have to configure something more in the switch in order use it as a network tap? (HowTos)