Gather Cisco Discovery Protocol informations with tcpdump

No cdpr at hand, no problem just throw this little tcpdump line at the interface of your interest:

tcpdump -nn -v -s 1500 -c 1 'ether[20:2] == 0x2000' -i eth0

If there are more than only CDP packets you may narrow it down to the Broadcast MAC:

tcpdump -nn -v -s 1500 -c 1 'ether[20:2] == 0x2000 and ether dst 01:00:0c:cc:cc:cc' -i eth0

Oh, and don't forget to wait for up to 60 seconds.