CDP

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## This will often show you the Cisco chassis switch, then use your firms asset management software to find the upstream switch.
## -s 1500 capture 1500 bytes of the packet (typical MTU size)
## ether[20:2] == 0x2000 - Capture only packets that are starting at byte 20, and have a 2 byte value of hex 2000 

tcpdump -v -s 1500 -c 1 'ether[20:2] == 0x2000'

LLDP

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## Switch:
tcpdump -i eth0 -s 1500 -XX -c 1 'ether proto 0x88cc'

## Port and CDP Neighbor Info:
tcpdump -v -s 1500 -c 1 '(ether[12:2]=0x88cc or ether[20:2]=0x2000)'

Info taken from