Roy's CCNA Notes

Day 36 - CDP & LLDP

Mar 08, 20254 min read

Jeremy’s IT Lab lecture video:

Day 36 - CDP & LLDP

Commands

Layer 2 Discovery Protocols Info

Layer 2 discovery protocols like Cisco Discovery Protocol (CDP) and Link Layer Discovery Protocol (LLDP) are used to share information and discover information about neighboring (directly connected) devices.

  • CDP is a Cisco proprietary protocol
  • LLDP is an industry standard protocol

The shared information can include Layer 2 and 3 data, like the host name, IP address, device type, etc.

They are usually considered to be security risks and are often not used in networks.

Layer 2 Discovery Protocols - CDP and LLDP

CDP and LLDP can both be run at the same time on the current networking device without any conflicts.

Cisco Discovery Protocol (CDP)

CDP Info

CDP is a Cisco proprietary protocol that is:

  • Enabled globally by default on Cisco devices, and enabled by default on each interface
  • Sends messages to MAC Address: 0100.0ccc.cccc
  • Timers:
    1. Message Timer: 60 seconds
    2. Holdtime: 180 seconds
  • CDPv2 messages are sent by default

When a device receives a CDP message, it is processed and discarded afterwards. The device will NOT forward it to other devices.

CDP Show

  • show cdp
    • Shows the global message timer, holdtimer, and CDP version
  • show cdp traffic
    • Shows traffic statistics about CDP packets
  • show cdp interface
    • Shows each interfaces’ encapsulation type, message timer, holdtimer
    • Gives a summary of CDP enabled interfaces at the end
  • show cdp neighbors
    • Shows a capability codes legend
    • Displays all the connected CDP devices along with their information, including: Device ID, locally connected interface, current holdtime, capabilities, platform and port ID
  • show cdp neighbors detail
    • Shows detailed connected CDP devices information, including: All the previous information, version, OS, CDP version, configured holdtime, VTP information, IP addresses of the neighboring connected interface, duplex mode, native VLAN.
  • show cdp entry HOST-NAME
    • Same as show cdp neighbors detail, but it shows information for the specified neighbor/host

CDP Config

  • cdp run
    • Enables CDP globally
  • cdp enable
    • Enables CDP on the current interface
  • cdp timer SECONDS 
    • Sets the CDP message timer
  • cdp holdtime SECONDS 
    • Sets the CDP message holdtime
  • cdp advertise-v2
    • Changes CDP version to V2

CDP Info

LLDP is an industry standard protocol that is:

  • Disabled globally by default on Cisco devices, so it must be manually enabled and also enabled on each interface
  • Sends messages to MAC Address: 0180.c200.000e
  • Timers:
    1. Message Timer: 30 seconds
    2. Holdtime: 120 seconds
    3. Reinitialization delay: 2 seconds
  • LLDP has an initialization timer (2 seconds) that starts counting once LLDP is enabled (globally or on an interface), the timer will delay the actual initialization of LLDP.

When a device receives an LLDP message, it is processed and discarded afterwards. The device will NOT forward it to other devices.

LLDP Show

  • show lldp
    • Shows the global message timer, holdtimer, and initialization timer
  • show lldp traffic
    • Shows traffic statistics about LLDP packets
  • show lldp interface
    • Shows each interfaces’ transmit, transmit state, receive and receive state
  • show lldp neighbors
    • Shows a capability codes legend
    • Displays all the connected LLDP devices along with their information, including: Device ID, locally connected interface, configured holdtime, capabilities, and port ID
  • show lldp neighbors detail
    • Shows detailed connected LLDP devices information, including: All the previous information, OS, current holdtime, system capabilities and enabled capabilities
  • show lldp entry HOST-NAME
    • Same as show lldp neighbors detail, but it shows information for the specified neighbor/host

CDP Config

  • lldp run
    • Enables LLDP globally
  • lldp transmit
    • Enables LLDP message transmission on the current interface
  • lldp receive
    • Enables LLDP message receiving on the current interface
  • lldp timer SECONDS 
    • Sets the LLDP message timer
  • lldp holdtime SECONDS 
    • Sets the LLDP message holdtime
  • lldp reinit SECONDS
    • Changes the LLDP reinitialization timer

Graph View

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