LLDP is an unidirectional protocol that uses the MAC layer to transmit specific information related to the capabilities and status of the local device. Separately from the transmit direction, the LLDP agent can also receive the same kind of information for a remote device which is stored in the related MIBs.
LLDP does not contain a mechanism for soliciting specific information from other LLDP agents, nor does it provide a specific means of confirming the receipt of information. LLDP allows the transmitter and the receiver to be separately enabled, making it possible to configure an implementation so the local LLDP agent can either transmit only or receive only, or can transmit and receive LLDP information.
The information fields in each LLDP frame are contained in a LLDP Data Unit (LLDPDU) as a sequence of variable length information elements, that each include type, length, and value fields (known as TLVs), where:
type identifies what kind of information is being sent
length indicates the length of the information string in octets
value is the actual information that needs to be sent (for example, a binary bit map or an alphanumeric string that can contain one or more fields)
Each LLDPDU contains four mandatory TLVs and can contain optional TLVs as selected by network management:
Chassis ID TLV
Port ID TLV
Time To Live TLV
Zero or more optional TLVs, as allowed by the maximum size of the LLDPDU
End Of LLDPDU TLV
The chassis ID and the port ID values are concatenated to form a logical identifier that is used by the recipient to identify the sending LLDP agent/port. Both the chassis ID and port ID values can be defined in a number of convenient forms. When selected however, the chassis ID/port ID value combination remains the same as long as the particular port remains operable.
A non-zero value in the TTL field of the Time To Live TLV tells the receiving LLDP agent how long all information pertaining to this LLDPDU identifier will be valid so that all the associated information can later be automatically discarded by the receiving LLDP agent if the sender fails to update it in a timely manner. A zero value indicates that any information pertaining to this LLDPDU identifier is to be discarded immediately.
Note that a TTL value of zero can be used, for example, to signal that the sending port has initiated a port shutdown procedure. The End Of LLDPDU TLV marks the end of the LLDPDU.
The implementation defaults to setting the port-id field in the LLDP OAMPDU to tx-local. This encodes the port-id field as ifIndex (sub-type 7) of the associated port. This is required to support some releases of SAM. SAM may use the ifIndex value to correctly build the Layer Two Topology Network Map. However, this numerical value is difficult to interpret or readily identify the LLDP peer when reading the CLI or MIB value without SAM. Including the port-desc option as part of the tx-tlv configuration allows an ALU remote peer supporting port-desc preferred display logic to display the value in the port description TLV instead of the port-id field value. This does not change the encoding of the port-id field. That value continues to represent the ifIndex. In some environments, it may be important to select the specific port information that is carried in the port-id field. The operator has the ability to control the encoding of the port-id information and the associated sub-type using the port-id-sub-type option. Three options are supported for the port-idsub-type:
tx-if-alias
Transmit the ifAlias String (sub-type 1) that describes the port as stored in the IFMIB, either user configured description or the default entry (ie 10/100/Gig Ethernet SFP)
tx-if-name
Transmits the ifName string (sub-type 5) that describes the port as stored in the IFMIB, ifName info
tx-local
The interface ifIndex value (sub-type 7)
IPv6 (address sub-type 2) and IPv4 (address sub-type 1) LLDP System Management addresses are supported.