Link Layer Discovery Protocol (LLDP)
Page 232 7750 SR OS Basic System Configuration Guide
Link Layer Discovery Protocol (LLDP)
The IEEE 802.1ab Link Layer Discovery Protocol (LLDP) is a uni-directional 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 MIB(s).
LLDP itself 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. Once 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’s 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’s 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.
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