CDP: layer 2 Cisco-proprietary protocol for devices to advertise themselves to neighbors.  
SNAP header required (Ethernet, Frame Relay, ATM etc?).  
Does not use/depend on IP, e.g. will work even if interface has no IP address
but will not work if interface is (shut)down.

Type of system, Pltaform, interfaces, IPs, IOS version, hostname.


show cdp
  entry        Information for specific neighbor entry
  interface    CDP interface status and configuration
  neighbors    CDP neighbor entries
     detail  
  traffic      CDP statistics

debug cdp
  adjacency    CDP neighbor info
  events       CDP events
  ip           CDP ip info
  packets      CDP packet-related information

#sh cdp
  Global CDP information:
        Sending CDP packets every 60 seconds
        Sending a holdtime value of 180 seconds

#sh cdp neighbors
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
                  S - Switch, H - Host, I - IGMP, r - Repeater

Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
umuc-pro2500        Eth 0/0        155           R        CPA2509   Eth 0

#sh cdp traffic
CDP counters :
        Packets output: 15, Input: 11
        Hdr syntax: 0, Chksum error: 0, Encaps failed: 0
        No memory: 0, Invalid packet: 0, Fragmented: 0