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For more information about LLDPDU TLVs, see the IEEE standard (LLDP) 802.1AB-2005 and the LLDP-MED
standard (ANSI/TIA-1057).
Management address
The management address of a device is used by the network management system to identify and
manage the device for topology maintenance and network management. The management address is
encapsulated in the management address TLV.
LLDP operating modes
LLDP can operate in one of the following modes:
• TxRx mode —A port in this mode sends and receives LLDPDUs.
•
Tx mode —A port in this mode only sends LLDPDUs.
•
Rx mode —A port in this mode only receives LLDPDUs.
• Disable mode —A port in this mode does not send or receive LLDPDUs.
Each time the LLDP operating mode of a port changes, its LLDP protocol state machine re-initializes. To
prevent LLDP from being initialized too frequently at times of frequent operating mode change, an
initialization delay, which is user configurable, is introduced. With this delay mechanism, a port must
wait for the specified interval before it can initialize LLDP after the LLDP operating mode changes.
How LLDP works
Transmitting LLDPDUs
An LLDP-enabled port operating in TxRx mode or Tx mode sends LLDPDUs to its directly connected
devices both periodically and when the local configuration changes. To prevent the network from being
overwhelmed by LLDPDUs at times of frequent local device information change, an interval is introduced
between two successive LLDPDUs.
This interval is shortened to 1 second in either of the following cases:
• A new neighbor is discovered. A new LLDPDU is received carrying device information new to the
local device.
•
The LLDP operating mode of the port changes from Disable/Rx to TxRx or Tx.
This is the fast sending mechanism of LLDP. With this mechanism, a specific number of LLDPDUs are sent
successively at the 1-second interval to help LLDP neighbors discover the local device as soon as possible.
Then, the normal LLDPDU transit interval resumes.
Receiving LLDPDUs
An LLDP-enabled port operating in TxRx mode or Rx mode checks the TLVs carried in every LLDPDU it
receives for validity violation. If valid, the information is saved and an aging timer is set for it based on
the TTL TLV carried in the LLDPDU. If the TTL TLV is zero, the information is aged out immediately.
Compatibility of LLDP with CDP
You must enable CDP compatibility for your device to work with Cisco IP phones.
As your LLDP-enabled device cannot recognize Cisco Discovery Protocol (CDP) packets, it does not
respond to the requests of Cisco IP phones for the voice VLAN ID configured on the device. This can