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The transmission and the reception of LLDPDUs can be separately enabled for every port,
making it possible to configure an implementation to restrict the port either to transmit only or
receive only, or to allow the port to both transmit and receive LLDPDUs. Four LLDP admin
statuses are supported by each port.
Tx&Rx: the port can both transmit and receive LLDPDUs.
Rx_Only: the port can receive LLDPDUs only.
Tx_Only: the port can transmit LLDPDUs only.
Disable: the port cannot transmit or receive LLDPDUs.
2) LLDPDU transmission mechanism
If the ports are working in TxRx or Tx mode, they will advertise local information by
sending LLDPDUs periodically.
If there is a change in the local device, the change notification will be advertised. To
prevent a series of successive LLDPDUs transmissions during a short period due to
frequent changes in local device, a transmission delay timer is set by network
management to ensure that there is a defined minimum time between successive
LLDP frame transmissions.
If the LLDP admin status of the port is changed from Disable/Rx to TxRx/Tx, the Fast
Start Mechanism will be active, the transmit interval turns to be 1 second, several
LLDPDUs will be sent out, and then the transmit interval comes back to the regular
interval.
3) LLDPDU receipt mechanism
When a port is working in TxRx or Rx mode, the device will check the validity of the received
LLDPDUs and the attached TLVs, save this neighbor information to the local device and then
set the aging time of this information according to the TTL value of TTL (Time To Live) TLV.
Once the TTL is 0, this neighbor information will be aged out immediately.
The aging time of the local information in the neighbor device is determined by TTL. Hold
Multiplier is a multiplier on the Transmit Interval that determines the actual TTL value used in an
LLDPDU. TTL = Hold Multiplier * Transmit Interval.
TLV
TLV refers to Type/Length/Value and is contained in a LLDPDU. Type identifies what kind of
information is being sent, Length indicates the length of information string in octets and Value
is the actual information to be sent. The basic TLV Format is shown as follows:
Each TLV is identified by a unique TLV type value that indicates the particular kind of
information contained in the TLV.
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