Two reasons a network administrator would use CDP

Verify Layer 2 connectivity between two devices when Layer 3 fails.

Obtain the IP address of a connected device in order to telnet to the device.

Cisco Discovery Protocol (CDP) is primarily used to obtain protocol addresses of neighboring devices and discover the platform of those devices. CDP can also be used to show information about the interfaces your router uses. CDP is an independent media protocol and runs on all Cisco-manufactured devices including routers, bridges, access servers, and switches. It should be noted that CDP is a protocol which works on the layer2. 

By default, multicast advertise is sent every 60 seconds to 01-00-0 c-cc-cc-cc as the destination address . When reaching the holdtime of 180 seconds , if not receiving the advertise from neighboring devices yet, the information of neighboring devices will be cleared.

Cisco Discovery Protocol (CDP) is a proprietary protocol designed by Cisco to help administrators collect information about both locally attached and remote devices. By using CDP, you can gather hardware and protocol information about neighbor devices, which is useful info for troubleshooting and documenting the network.

Router1#Show cdp neighbor

Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
                              S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone

Device ID        Local Intrfce       Holdtme         Capability       Platform          Port ID
Router1              Fas 0/1              141                   R               C1841            Fas 0/1

Router1#Show cdp neighbor details 

Device ID: Router2
Entry address(es):
  IP address : 192.168.100.1
Platform: cisco C1841, Capabilities: Router
Interface: FastEthernet0/1, Port ID (outgoing port): FastEthernet0/1
Holdtime: 162

Version :
Cisco IOS Software, 1841 Software (C1841-ADVIPSERVICESK9-M), Version 12.4(15)T1, RELEASE SOFTWARE
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2007 by Cisco Systems, Inc.
Compiled Wed 18-Jul-07 04:52 by pt_team

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