LLDP Work Mechanism

LLDP Work Mode

The port includes the following four LLDP work modes:

• RxTx: send and receive LLDPDU;
• Tx: only send LLDPDU;
• Rx: only receive LLDPDU;
• Disable: do not send or receive LLDPDU.

LLDP Sending Mechanism

The LLDP sending mechanism:

• When the port works in the RxTx or Tx mode, regularly send LLDPDU to the neighbor device according to the sending period of the LLDP packet;
• After the port enables the polling function, regularly poll whether the LLDP concerned configuration in the local device changes. If the configuration changes, send LLDPDU at once. To prevent the frequent change of the local information from causing lots of the sent LLDPDU, it is necessary to delay and wait for some time and then continue to send the next LLDPDU when sending one LLDPDU every time.
• When some configuration related with the local device LLDP changes (for example, select the released TLV type), or if finding the configuration change after enabling the polling function, enable the fast sending mechanism, that is, immediately send the LLDPDU of the specified quantity continuously, and then restore the normal LLDP packet sending period.
• When the global LLDP function is disabled or the port enabled with LLDP executes shutdown, adds to the aggregation group, and disables the LLDP, as well as restarts the device, send one LLDPDU with CLOSE TLV to inform the neighbor device.

LLDP Receiving Mechanism

When the port works in the RxTx or Rx mode, check the validity of the received LLDPDU and the carried TLV. After passing the validity check, save the neighbor information to the local device and set the age time of the neighbor information at the local device according to the TTL (Time To Live) carried in LLDPU. If the TTL value in the received LLDPDU is 0, age the neighbor information at once. The storing capability of the LLDP protocol for the neighbor is limited. If the neighbors reach the threshold, more neighbor advertising packets are dropped and cannot be saved.