Day 36: CDP and LLDP

CCNA 200-301 Study Guide: Layer 2 Discovery (CDP & LLDP)
1.0 Fundamentals of Layer 2 Discovery
Layer 2 discovery protocols act as the "digital handshake" of a network, allowing devices to identify neighbors without requiring an IP address. They operate at the Data Link Layer, meaning discovery occurs as long as the physical link is up and the protocol is enabled.
Core Attributes
Operational Scope: Information is strictly point-to-point. Discovery frames are processed by the direct neighbor and then discarded; they are never forwarded.
Shared Data: Device IDs (Hostnames), IP addresses (for management), platform/capabilities, and local/remote interface mappings.
Security Risk: These protocols are "chatty" and broadcast sensitive topology data. Standard Practice: Disable discovery on untrusted ports (e.g., user-facing access ports) to prevent reconnaissance.
2.0 Cisco Discovery Protocol (CDP)
CDP is a Cisco-proprietary protocol used for identifying and managing Cisco hardware.
Operational Defaults
Status: Enabled globally and on all interfaces by default.
Multicast MAC: 0100.0CCC.CCCC
Default Timers:
Hello Timer: 60 seconds.
Holdtime: 180 seconds.
Version 2 (Default): Includes enhanced features like Native VLAN and duplex mismatch detection.
Configuration & Management
Command
Mode
Purpose
cdp run
Global
Enables CDP globally.
no cdp run
Global
Disables CDP globally.
cdp enable
Interface
Enables CDP on a specific port.
cdp timer [sec]
Global
Adjusts advertisement frequency.
cdp holdtime [sec]
Global
Adjusts neighbor retention time.
Verification Commands
show cdp: Checks global status and timers.
show cdp neighbors: Provides a summary table of adjacent Cisco devices.
show cdp neighbors detail: Provides Layer 3 addresses and software versions.
show cdp entry [name]: Focuses on a specific neighbor.
3.0 Link Layer Discovery Protocol (LLDP)
LLDP (IEEE 802.1AB) is the vendor-neutral standard required for discovery in multi-vendor environments.
Operational Defaults
Status: Typically disabled by default on Cisco switches.
Multicast MAC: 0180.C200.000E
Default Timers:
Hello Timer: 30 seconds.
Holdtime: 120 seconds.
Reinit Delay: 2 seconds.
Configuration & Management
Unlike CDP, LLDP allows for granular control over transmission and reception.
Command
Mode
Purpose
lldp run
Global
Enables LLDP globally.
lldp transmit
Interface
Enables sending LLDP frames on the port.
lldp receive
Interface
Enables processing incoming LLDP frames.
lldp timer [sec]
Global
Configures advertisement frequency.
lldp holdtime [sec]
Global
Configures the holdtime.
Verification Commands
show lldp
show lldp neighbors
show lldp neighbors detail
Note: LLDP uses B (Bridge) in its capability codes where CDP uses S (Switch).
4.0 Protocol Comparison Matrix
Feature
Cisco Discovery Protocol (CDP)
Link Layer Discovery (LLDP)
Ownership
Cisco Proprietary
IEEE 802.1AB (Standard)
Cisco Default
Enabled
Disabled
Hello Timer
60 Seconds
30 Seconds
Holdtime
180 Seconds
120 Seconds
Interface Logic
cdp enable (Binary)
transmit / receive (Granular)
VTP Support
Yes
No
Capability Code
S (Switch)
B (Bridge)