Well that was easy.
Ubiquiti EdgeRouter X SFP can mirror ports: Go to Config Tree->Interfaces->Ethernet->eth0 and put the destination port into the 'mirror field'.
Plug in a laptop, disable dhcp on the nic, and run: `sudo tcpdump -i ethf00 -n -X` and presto, packets!
Reportedly this is a software copy so router CPU usage will increase, but I don't see it going over 30% right now.
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