Working mechanism
LLDP operating modes
An LLDP agent can operate in one of the following modes:
TxRx mode—An LLDP agent in this mode can send and receive LLDP frames.
Tx mode—An LLDP agent in this mode can only send LLDP frames.
Rx mode—An LLDP agent in this mode can only receive LLDP frames.
Disable mode—An LLDP agent in this mode cannot send or receive LLDP frames.
Each time the LLDP operating mode of an LLDP agent changes, its LLDP protocol state machine reinitializes. A configurable reinitialization delay prevents frequent initializations caused by frequent changes to the operating mode. If you configure the reinitialization delay, an LLDP agent must wait the specified amount of time to initialize LLDP after the LLDP operating mode changes.
Transmitting LLDP frames
An LLDP agent operating in TxRx mode or Tx mode sends LLDP frames to its directly connected devices both periodically and when the local configuration changes. To prevent LLDP frames from overwhelming the network during times of frequent changes to local device information, LLDP uses the token bucket mechanism to rate limit LLDP frames. For more information about the token bucket mechanism, see ACL and QoS Configuration Guide.
LLDP automatically enables the fast LLDP frame transmission mechanism in either of the following cases:
A new LLDP frame is received and carries device information new to the local device.
The LLDP operating mode of the LLDP agent changes from Disable or Rx to TxRx or Tx.
The fast LLDP frame transmission mechanism successively sends the specified number of LLDP frames at a configurable fast LLDP frame transmission interval. The mechanism helps LLDP neighbors discover the local device as soon as possible. Then, the normal LLDP frame transmission interval resumes.
Receiving LLDP frames
An LLDP agent operating in TxRx mode or Rx mode confirms the validity of TLVs carried in every received LLDP frame. If the TLVs are valid, the LLDP agent saves the information and starts an aging timer. The initial value of the aging timer is equal to the TTL value in the Time To Live TLV carried in the LLDP frame. When the LLDP agent receives a new LLDP frame, the aging timer restarts. When the aging timer decreases to zero, all saved information ages out.