From CCIE Study Wiki
CDP is a Cisco-proprietary Layer 2 protocol that allows the device to discover other directly connected devices and learn how they are configured.
Theory
CDP messages are listened for and sent to the multicast MAC of 0100:0CCC:CCCC.
CDP advertisements contain TTL info, which indicates the length of time a receiving device should hold CDP info before discarding it. A receiver stores CDP info in a table that is refreshed each time an advertisement is received. The info is discarded after three advertisements are missed.
SNMP will interact with CDP by using CDP info to learn the device type and SNMP agent address of neighboring devices.
Commands
- no cdp run (global) - turns off CDP globally on a device
- no cdp enable (interface) - turns CDP off on the specific interface
- cdp timer seconds (global) - specifies the transmission frequency of CDP packets
- cdp holdtime seconds (global) - specifies the amount of time a receiving device should hold CDP info before discarding it
Default Settings
- CDP is on by default
- CDP messages sent every 60 seconds
- CDP holdtime is 180 seconds
Verification
Troubleshooting, Tips, and Tricks
- Not supported on Frame Relay multipoint subinterfaces.
Online Resources
- Cisco IOS Network Management Configuration Guide section on CDP.
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