LLDP is invaluable for network administrators for:

• Network topology discovery: Quickly mapping out network connections and understanding physical connectivity.

• Troubleshooting: Identifying connectivity issues, verifying physical connections, and detecting misconfigurations.

• Asset management: Gaining visibility into connected devices and their attributes, aiding in inventory and documentation.

• Deployment and configuration: Informing other protocols or automated systems, particularly for Power over Ethernet (PoE) devices, to ensure proper power delivery and network policy application.

LLDP, while highly beneficial, has specific operational characteristics and potential considerations:

• Layer 2 only operation: LLDP operates strictly at Layer 2 (the data link layer) of the OSI model. This means it can only discover and exchange information with directly connected neighbors on the same physical segment. It does not provide information about devices across Layer 3 boundaries.

• Information disclosure: LLDP broadcasts device information, which could potentially be leveraged by unauthorized parties if enabled on untrusted or publicly accessible ports. It is not an authentication mechanism and does not verify the identity of the neighbor.

• Resource consumption: Although generally minimal, enabling LLDP on a large number of ports, especially in very dense environments, can consume a small amount of CPU and memory resources for processing and storing neighbor information.

• No configuration enforcement: LLDP is a discovery protocol; it gathers and advertises information. It does not automatically apply or propagate configurations to neighboring devices. Other protocols or manual configurations are required to act upon the discovered information.

• Vendor-specific TLVs: While LLDP is an open standard, vendors may implement proprietary TLVs for specific features or information. These vendor-specific TLVs might not be understood or processed by devices from other manufacturers.

• Unidirectional operation: If LLDP is configured in send-only or receive-only mode on a port, the full discovery capability between two devices might be limited. For complete neighbor discovery, both devices should be configured to send and receive LLDP packets.

LLDP is primarily a single protocol, but its functionality can be extended, and it operates in different modes.

• Base LLDP: This refers to the core LLDP protocol (IEEE 802.1AB) that provides the fundamental device discovery and information exchange capabilities. It includes the standard TLVs for chassis ID, port ID, system name, system description, and capabilities.

• LLDP-MED (Media Endpoint Devices): This is an extension to the base LLDP standard (ANSI/TIA-1057) designed specifically for Voice over IP (VoIP) phones, video conferencing units, and other media endpoint devices. LLDP-MED provides additional TLVs that are crucial for these applications, such as:

  • Network policy: Advertises VLAN IDs, Layer 2 priority (802.1p), and Layer 3 Differentiated Services Code Point (DSCP) values for voice and video traffic.

  • Power over MDI (Media Dependent Interface): Provides detailed power management information for PoE devices, including power type, source, priority, and allocated power.

  • Inventory: Allows discovery of hardware and software versions of the endpoint.

  • Location identification: Enables the endpoint to send its physical location (e.g., civic address, coordinate-based location) to the switch, which is critical for emergency services (E911).

• LLDP modes of operation: On Cisco switches, LLDP can be configured to operate in different modes on a per-interface basis:

  • Transmit (Tx): The interface sends LLDPDUs but does not process incoming LLDPDUs.

  • Receive (Rx): The interface processes incoming LLDPDUs but does not send its own LLDPDUs.

  • Transmit and Receive (TxRx): The interface sends and receives LLDPDUs. This is typically the default and recommended mode for full discovery.

  • Disabled: LLDP is not active on the interface.