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Layer 2 Discovery Protocols Overview

Jun 19, 2025

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Overview

This lecture covers Layer 2 discovery protocols—CDP (Cisco Discovery Protocol) and LLDP (Link Layer Discovery Protocol)—their functions, configurations, and command usage relevant to the CCNA exam.

Layer 2 Discovery Protocols Overview

  • Layer 2 discovery protocols allow directly connected devices to exchange information like host name, device type, and IP address.
  • They operate at Layer 2 of the OSI model using MAC addresses instead of IP addresses.
  • CDP is Cisco proprietary; LLDP is an industry standard (IEEE 802.1AB).
  • These protocols can be security risks due to the information they share.
  • Typically used for network troubleshooting and topology discovery.

Cisco Discovery Protocol (CDP)

  • CDP is enabled by default on most Cisco devices.
  • CDP messages are sent every 60 seconds to multicast MAC address 0100.0CCC.CCCC.
  • Only directly connected neighbors receive and process CDP messages.
  • Default holdtime for CDP neighbors is 180 seconds.
  • Key CDP show commands: SHOW CDP, SHOW CDP TRAFFIC, SHOW CDP INTERFACE, SHOW CDP NEIGHBORS, SHOW CDP NEIGHBORS DETAIL, SHOW CDP ENTRY.
  • SHOW CDP NEIGHBORS displays device ID, local interface, holdtime, capabilities (R for router, S for switch), platform/model, and port ID.
  • SHOW CDP NEIGHBORS DETAIL provides additional info such as software version, VTP information, native VLAN, duplex setting, and neighbor IP address.
  • CDP can be enabled/disabled globally (CDP RUN/NO CDP RUN) or per interface (CDP ENABLE/NO CDP ENABLE).
  • Timers can be adjusted with CDP TIMER and CDP HOLDTIME commands.

Link Layer Discovery Protocol (LLDP)

  • LLDP is disabled by default on Cisco devices and must be manually enabled.
  • LLDP messages are sent every 30 seconds by default to multicast MAC address 0180.C200.000E.
  • Default LLDP holdtime is 120 seconds; reinitialization delay is 2 seconds.
  • Enable LLDP globally with LLDP RUN; per-interface transmission and reception enabled with LLDP TRANSMIT and LLDP RECEIVE.
  • LLDP timers are configured with LLDP TIMER, LLDP HOLDTIME, and LLDP REINIT.
  • Key LLDP show commands: SHOW LLDP, SHOW LLDP TRAFFIC, SHOW LLDP INTERFACE, SHOW LLDP NEIGHBORS, SHOW LLDP NEIGHBORS DETAIL, SHOW LLDP ENTRY.
  • LLDP uses B (Bridge) for switches, R for routers in system capabilities.
  • LLDP cannot report VTP information but can show OS version.

Commands & Exam Tips

  • For CDP: use SHOW CDP and SHOW CDP INTERFACE to see configured timers.
  • Default CDP state: CDP ENABLE on interfaces and CDP TIMER 60.
  • SHOW CDP NEIGHBORS gives device ID, local and neighbor interface, capabilities, and platform/model.
  • For LLDP, both TRANSMIT and RECEIVE must be enabled per interface.

Key Terms & Definitions

  • CDP (Cisco Discovery Protocol) — Cisco-proprietary Layer 2 protocol for device discovery.
  • LLDP (Link Layer Discovery Protocol) — Vendor-neutral, industry standard Layer 2 discovery protocol.
  • Holdtime — Duration a device retains neighbor information without receiving updates.
  • Capability Codes — Letters identifying device roles (R = Router, S = Switch, B = Bridge).
  • Multicast MAC Address — A special MAC address used to send messages to multiple devices simultaneously.

Action Items / Next Steps

  • Review the Anki flashcards on protocol MAC addresses.
  • Complete the Packet Tracer lab for hands-on practice (available in next video).
  • Memorize key show commands and their outputs for CDP and LLDP.
  • Read supplementary materials if provided (via mailing list or course links).

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