User's Manual

289
Chapter 40 Link Layer 2 Discovery Protocol
(LLDP)
40.1 LLDP Overview
802.1ABThe link layer discovery protocol (LLDP) at 802.1AB helps to detect network troubles easily and
maintain the network topology. It enables the neighboring device to sending out notice of its own state
information to other devices and each port of all devices stores information of defining themselves. If necessary,
they can also sending update information to the neighboring devices and the neighboring devices will store
the information in standard SNMP MIBs. The network management system can inquire the connection of
current layer-2 from MIB. LLDPcan neither configure nor control the network element or traffic. It only reports
configuration of layer-2.
Simply, LLDP is a neighbor discovery protocol. It sets a standard method for the Ethernet network device,
such as switches, routers and WAPs. It enables the Ethernet device notify its existence to other nodes and
save the discovery information of neighboring devices. For instance, all information including the device
configuration and the device identification can be notified through the protocol. Specifically, LLDP defines a
universal notification information set, a transmission notification protocol and a method of storing all notification
information. The device need to notify the notification information can transmit many notifications in a LAN data
packet. The transmission type is TLV.
TLV has three compulsory types: Chassis ID TLV, Port ID TLV and Time To Live TLV; five optional types:
Port Description, System Name, System Description, System Capabilities and Management Address; and
three extension TLVs: DOT1 (Port Vlan ID, Protocol Vlan ID, Vlan Name, Protocol Identity); DOT3 (MAC/PHY
Configuration/Status, Power Via MDI, Link Aggregation, Max Frame Size); MED (MED Capability, Network
Policy, Location Identification, Extended Power-via-MDI, Inventory (Hardware Revision, Firmware Revision,
Software Revision, Serial Number, Manufacturer Name, Mode Name, Assert ID).
LLDP is a unidirectional protocol. One LLDP agent transmits its state information and functions through
its connected MSAP, or receives the current state information or function information about the neighbor.
However, the LLDP agent cannot request any information from the peer through the protocol. During message
exchange, message transmission and reception do not affect each other. You can configure only message
transmission or reception or both.
40.2 Initializing the Protocol
LLDP can work under three modes: transmit-only, receive-only and transmit-and-receive. The default
mode is transmit-and-receive.
40.2.1 Initializing LLDP Transmit Mode
Set LLDP to transmit-only in the interface mode. In transmit-only mode, the interface transmits LLDP
packets when the state or value of one or more information elements (management object) of the local system
change or the transmission timer is timeout. The interface will not transmit LLDP packets when disabling the