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Day 36 - CDP & LLDP

Reviewed: No

1. Introduction to Layer 2 Discovery Protocols

  • Purpose: Used to share information between directly connected devices, such as hostname, IP address, and device type.
  • Layer 2 Operation: These protocols function at Layer 2, meaning they don't use IP addresses.
  • Protocols:
    • CDP (Cisco Discovery Protocol): Cisco proprietary protocol.
    • LLDP (Link Layer Discovery Protocol): Industry-standard protocol, IEEE 802.1AB.

2. CDP (Cisco Discovery Protocol)

  • Default State:
    • Enabled by default on Cisco devices (routers, switches, firewalls, IP phones).
    • Uses multicast MAC address 0100.0CCC.CCCC.
  • Message Timers:
    • Message Interval: 60 seconds.
    • Holdtime: 180 seconds (entry removed if no message is received in this time).
    • Version: CDP v2 is used by default.

CDP CLI Commands:

  • View CDP Status:
    • show cdp – Shows CDP status, including message interval and holdtime.
    • show cdp traffic – Displays CDP packet statistics.
    • show cdp interface – Shows CDP status per interface, including message timer and holdtime.
  • View Neighbor Information:
    • show cdp neighbors – Lists CDP neighbors and their details.
    • Columns:
      • Device ID: Hostname of the neighbor.
      • Local Interface: Interface on this device (e.g., G0/0).
      • Holdtime: Time before the neighbor is removed.
      • Capabilities: Type of device (e.g., R for Router, S for Switch).
      • Platform: Model of the neighboring device.
      • Port ID: Interface on the neighboring device.
  • Detailed Neighbor Information:
    • show cdp neighbors detail – Displays additional details like software version, IP address, native VLAN, and duplex settings.
    • show cdp entry <neighbor> – Shows detailed information for a specific neighbor.

CDP Configuration Commands:

  • Enable/Disable CDP:
    • Global: cdp run to enable, no cdp run to disable.
    • Interface: cdp enable to enable, no cdp enable to disable.
  • Configure Timers:
    • Message Timer: cdp timer <seconds> (default 60).
    • Holdtime: cdp holdtime <seconds> (default 180).

3. LLDP (Link Layer Discovery Protocol)

  • LLDP Overview:
    • Industry-standard protocol supported by various vendors (Cisco, Juniper, Palo Alto).
    • Disabled by default on Cisco devices.
    • Multicast MAC Address: 0180.C200.000E.
    • Message Interval: 30 seconds.
    • Holdtime: 120 seconds.
  • Additional Timer:
    • Reinitialization Timer: Prevents rapid enable/disable (default 2 seconds).

LLDP CLI Commands:

  • View LLDP Status:
    • show lldp – Shows LLDP status and timers.
    • show lldp traffic – Displays LLDP packet statistics.
    • show lldp interface – Shows LLDP status for each interface (Tx and Rx).
  • View Neighbor Information:
    • show lldp neighbors – Lists LLDP neighbors.
    • show lldp neighbors detail – Displays detailed neighbor information, similar to CDP.
    • show lldp entry <neighbor> – Shows detailed information for a specific LLDP neighbor.

LLDP Configuration Commands:

  • Enable/Disable LLDP:
    • Global: lldp run to enable, no lldp run to disable.
    • Interface:
      • lldp transmit – Enable transmission of LLDP messages.
      • lldp receive – Enable reception of LLDP messages.
  • Configure Timers:
    • Message Timer: lldp timer <seconds> (default 30).
    • Holdtime: lldp holdtime <seconds> (default 120).
    • Reinitialization Timer: lldp reinit <seconds> (default 2).

4. CDP vs. LLDP:

  • CDP: Cisco proprietary, enabled by default on Cisco devices, shares more detailed information (e.g., VTP settings).
  • LLDP: Industry-standard, requires manual configuration on Cisco devices, compatible with non-Cisco devices.

5. Best Practices:

  • Security Risk: Both protocols can share sensitive network information (hostname, IP, etc.). Many admins disable them to reduce security risks.
  • Use Cases: CDP is useful in all-Cisco environments, while LLDP is necessary in multi-vendor environments.

6. Wireshark Captures:

  • CDP Capture: Shows the multicast MAC address, CDP version, and detailed device information.
  • LLDP Capture: Shows similar information but uses a different multicast MAC address and slightly different fields.

Quiz Summary:

  1. CDP Timers:
    • Commands: show cdp and show cdp interface display CDP timers.
  2. CDP Default State:
    • CDP enabled on interfaces by default (cdp enable) and uses a 60-second message timer.
  3. LLDP Capabilities:
    • A multilayer switch (SW1) shows system capabilities B, R (Bridge and Router).
  4. LLDP Configuration:
    • LLDP requires separate lldp transmit and lldp receive commands for each interface.
    • LLDP can be used to discover the OS version of a neighboring device.