With Link Layer Discovery Protocol (LLDP) you can obtain node and topology information to help detect and correct network and configuration errors.
802.1AB is the IEEE standard called Station and Media Access Control Connectivity Discovery. This standard defines the Link Layer Discovery Protocol.
LLDP stations connected to a local area network (LAN) can advertise station capabilities to each other, allowing the discovery of physical topology information for network management.
LLDP-compatible stations can comprise any interconnection device, including PCs, IP Phones, switches, and routers.
Each LLDP station stores LLDP information in a standard Management Information Base (MIB), making it possible for a network management system (NMS) or application to access the information.
The functions of an LLDP station include:
Advertising connectivity and management information about the local station to adjacent stations
Receiving network management information from adjacent stations
Enabling the discovery of certain configuration inconsistencies or malfunctions that can result in impaired communications at higher layers
For example, you can use LLDP to discover duplex mismatches between an IP Phone and the connected switch.
LLDP is compatible with IETF PROTO MIB (IETF RFC 2922).
The following figure shows an example of a LAN using LLDP.
Legend:
The switch and an LLDP-enabled router advertise chassis and port IDs and system descriptions to each other
The devices store the information about each other in local MIB databases, accessible with SNMP
A network management system retrieves the data stored by each device and builds a network topology map
Switch
Router
Management work station
IP Phone
LLDP is a one-way protocol.
An LLDP agent can transmit information about the capabilities and current status of the system associated with its MAC service access point (MSAP) identifier.
The LLDP agent also can receive information about the capabilities and current status of the system associated with a remote MSAP identifier.
However, LLDP agents cannot solicit information from each other.
You can configure the local LLDP agent to transmit and receive.
The information parameters in each LLDP frame are in a LLDP Protocol Data Unit (PDU) as a sequence of short, variable length information elements known as TLVs (type, length, value).
Each LLDP PDU includes the following mandatory TLVs:
Chassis ID
Port ID
Time To Live
Port Description
System Name
System Description
System Capabilities (indicates both the system supported capabilities and enabled capabilities, such as end station, bridge, or router)
Management Address
The chassis ID and the port ID values are concatenated to form a logical MSAP identifier that the recipient uses to identify the sending LLDP agent and port.
A non-zero value in the Time to Live (TTL) field of the TTL TLV indicates to the receiving LLDP agent how long the LLDP PDU information from the MSAP identifier remains valid.
The receiving LLDP agent automatically discards all LLDP PDU information, if the sender fails to update it in a timely manner.
A zero value in TTL field of Time To Live TLV tells the receiving LLDP agent to discard the information associated with the LLDP PDU MSAP identifier.
LLDP and SONMP both advertise the same topology IP address for the Segmented Management Instance management interface. LLDP supports IPv4 and IPv6 advertisement. If all three management interfaces are configured, the advertised default topology IP priority is management CLIP, then management VLAN, then management OOB. You can change the default topology IP using CLI or EDM. If multiple IPv4 addresses are configured on an OOB or VLAN management interface, the advertised IP priority is static IP address, then DHCP IP address, then link-local IP address.
When a transmit cycle is initiated, the LLDP manager extracts the managed objects from the LLDP local system MIB and formats this information into TLVs. TLVs are inserted into the LLDP PDU.
LLDP PDUs are regularly transmitted at a user-configurable transmit interval (tx-interval) or when any of the variables in the LLPDU is modified on the local system; for example, system name or management address.
Transmission delay (tx-delay) is the minimum delay between successive LLDP frame transmissions.
The LLDP local system MIB stores the information to construct the various TLVs for transmission.
The LLDP remote systems MIB stores the information received from remote LLDP agents.
The system discards LLDP PDUs and TLVs that contain detectable errors.
The system assumes that TLVs that contain no basic format errors, but that it does not recognize, are valid and stores them for retrieval by network management.
You must apply TLVs on a per-port basis.
Because LLDP manages trunked ports individually, TLVs configured on one port in a trunk do not propagate automatically to other ports in the trunk.
And the system sends advertisements to each port in a trunk, not on a per-trunk basis.
Fabric Attach uses LLDP to signal a desire to join the SPB network. When a switch is enabled as an FA Server, it receives IEEE 802.1AB LLDP messages from FA Client and FA Proxy devices requesting the creation of Switched UNI service identifiers (I-SIDs). All of the discovery handshakes and I-SID mapping requests are using LLDP TLV fields. Based on the LLDP standard, FA information is transmitted using organizational TLVs within LLDP PDUs.
FA also leverages LLDP to discover directly connected FA peers and to exchange information associated with FA between those peers.