Updated August 10, 2023
Introduction to LLDP
Link Layer Discovery Protocol or LLDP is used in network devices to know the network’s identity, capabilities, and other devices based on IEEE technology. LLDP is a standard used in layer 2 of the OSI model. This is enabled in default mode, and all supported interfaces send and receive LLDP packets from the networks. LLDP communicates with other devices and shares information with other devices. LLDP is used mainly to identify neighbors in the network so that security risks can be exposed. Destination address and cyclic redundancy check are used in LLDP frames.
Where Does it Fit in the Network Layer?
LLDP fits in the data link layer in level 2 of the standard network architecture subscribed by the OSI (Open Systems Interconnection) model. This model prescribed by the International Organization for Standardization deals with protocols for network communication between heterogeneous systems. In the OSI model, Information communication between 2 devices across the network is split into 7 layers, and they are bundled over one another in a sequence, and the layers are
| Sl | Description | Level | Function | Responsibility |
| 1 | Application layer | 7 | Process requests of End users and return results to them | Host |
| 2 | Presentation layer | 6 | Manage Encryption and decryption of data | Host |
| 3 | Session layer | 5 | Manage sessions | Host |
| 4 | Transport layer | 4 | Manage Delivery, Splitting the data as segments and reassembling | Host |
| 5 | Network layer | 3 | Create pockets from segments and vice versa. Manage pocket transfer across neighbor networks. | Network |
| 6 | Datalink layer | 2 | Create Data frames from Pockets and move the frames to other nodes within the same network (LAN & WAN) | Network |
| 7 | Physical layer | 1 | Provide a physical medium for data exchange | Network |
LLDP provides a standard protocol for moving the data frames (as part of the data link layer) created from the data pockets (sent by the network layer) and controls the transfer.
Protocol
LLDP protocol stipulates a standard set of rules and regulations for interaction between network devices in a multiple-vendor environment. It covers mainly how a device identifies itself and publicizes its’ capabilities in a network by transmitting a pack of information about itself periodically so that other devices can recognize it.
The information pack is part of the message in network frames (Ethernet frames) transmitted across network nodes. IEEE 802.1AB protocol is used in LLDP and is a vendor-neutral standard protocol.
Structure of LLDP Frames
The pack of information called an LLDP data unit follows a type length and value structure (TLV), and the following table lists the details of the information and its type of TLV.
| Type | Description | Mandatory or Optional TLVs |
| 1 | Identification of the device (Chassis ID) | Mandatory |
| 2 | Port Identification (Port ID) | Mandatory |
| 3 | Validity time of the received information | Mandatory |
| 4 | Port details like Description etc., | Optional and additional TLVs |
| 5 | Device name | Optional and additional TLVs |
| 6 | Software version | Optional and additional TLVs |
| 7 | Device function and its capabilities | Optional and additional TLVs |
| 8 | IP & MAC address of the device | Optional and additional TLVs |
| 0 | The signal indicating End of the details also the end of Frame | Mandatory |
Types and Operations of LLDPDU
The above LLDP data unit, which publishes information on one device to another neighbor device, is called a normal LLDPDU. When a port is disabled or shuts down, or rebooted, a shutdown advisory, LLDPU, is published to receiving devices indicating the LLDP signals are invalid after that.
There are 3 ways it can operate, and they are
- Transmit Only: It can transmit the information package about the local device.
- Receive only: It can only get the details of the remote device.
- Transmit & Receive: It does both of the above functions.
Whenever the data units are received from a remote device, mandatory and optional Time, length, and values are validated for correctness and dropped if errors occur.
Database of LLDP Information
The information about the LLDP data unit is stored in a management information database (MIB) both on the sending and receiving side. This information is used for network management purposes, and the data can be retrieved later using standard queries.
Extension to VOIP devices
LLD protocol can be extended to manage smartphones, IP phones, and other mobile devices to receive and send information over the network. Initially, it will start with sending raw LLDP data pockets, and once it senses the device on the other side is VOIP, it will send data pockets in LLDP-MED protocol till the communication is completed. There are separate times, lengths, and values for LLDP-MED protocols.
LLDP Management
The following time parameters are managed in LLDP with default values.
- Time duration upto which a device will retain the information about the pairing device before purging it
- Time gap to send the LLDP updates to the pairing device
- Delay time to initialize
Other aspects of LLDP Management are
- Configuration settings of network components
- Activation and deactivation of network components
- Maintenance of LLDP Characteristics
- Monitoring LLDP functionalities
Benefits of LLDP
- Managing a complex multiple-vendor network made it simple, structured, and easier.
- The accurate information captured on the data exchange helps control the network performance, monitor the data exchange flow, and troubleshoot issues whenever it occurs.
- Provides Better traceability of network components within the network
- Improves the system available to the users by effectively monitoring the network performance and preventing downtime in data center operations.
- Ensures good front-end response to users in the application by ensuring faster and quicker availability of data from other nodes in the same network and from other networks.
Conclusion
LLD protocol is a boon to network administrators. It aids them with helpful information on intra-network devices at the data layer (level 2) and the internetwork devices at the network layer (level 3) for effectively managing data center operations.
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