The IEEE 802.1ab Link Layer Discovery Protocol (LLDP) is a 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 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. After 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 is 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.
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 a LLDPDU TLV marks the end of the LLDPDU.
The IEEE 802.1ab standard defines a protocol that:
Advertises connectivity and management information about the local station to adjacent stations on the same IEEE 802 LAN.
Receives network management information from adjacent stations on the same IEEE 802 LAN.
Operates with all IEEE 802 access protocols and network media.
Establishes a network management information schema and object definitions that are suitable for storing connection information about adjacent stations.
Provides compatibility with a number of MIBs as depicted in Figure 1.
Figure 1. LLDP Internal Architecture for a Network Node
Network operators must be able to discover the topology information to detect and address network problems and inconsistencies in the configuration. Moreover, standard-based tools can address the complex network scenarios where multiple devices from different vendors are interconnected using Ethernet interfaces.
The example displayed in Figure 2 depicts a MPLS network that uses Ethernet interfaces in the core or as an access/hand off interfaces to connect to different kind of Ethernet enabled devices such as service gateway/routers, QinQ switches, DSLAMs or customer equipment.
IEEE 802.1ab LLDP running on each Ethernet interfaces in between all the above network elements may be used to discover the topology information.