[root@yourbox ~] # sudo tcpdump -nn -v -i eth1 -s 1500 -c 1 ‘ether[20:2] == 0x2000’

tcpdump: listening on eth1, link-type EN10MB (Ethernet), capture size 1500 bytes
13:53:51.808172 CDPv2, ttl: 180s, checksum: 692 (unverified), length 449
Device-ID (0x01), length: 26 bytes: ‘data_switch_3.dcctools.com’
Version String (0x05), length: 242 bytes:
Cisco IOS Software, C3560 Software (C3560-IPBASEK9-M), Version 12.2(55)SE, RELEASE SOFTWARE (fc2)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2010 by Cisco Systems, Inc.
Compiled Sat 07-Aug-10 22:26 by prod_rel_team
Platform (0x06), length: 20 bytes: ‘cisco WS-C3560G-48TS’
Address (0x02), length: 13 bytes: IPv4 (1) 192.168.222.254
Port-ID (0x03), length: 19 bytes: ‘GigabitEthernet0/10’
Capability (0x04), length: 4 bytes: (0x00000008): L2 Switch
Protocol-Hello option (0x08), length: 32 bytes:
VTP Management Domain (0x09), length: 3 bytes: ‘ctc’
Native VLAN ID (0x0a), length: 2 bytes: 15
Duplex (0x0b), length: 1 byte: full
AVVID trust bitmap (0x12), length: 1 byte: 0x00
AVVID untrusted ports CoS (0x13), length: 1 byte: 0x00
Management Addresses (0x16), length: 13 bytes: IPv4 (1) 192.168.222.254
unknown field type (0x1a), length: 12 bytes:
0x0000:  0000 0001 0000 0000 ffff ffff
1 packets captured
1 packets received by filter
0 packets dropped by kernel