4000 Router Series Layer 2 - LAN Switching Configuration Guide

Management address

The network management system uses the management address of a device to identify and manage the

device for topology maintenance and network management. The management address is encapsulated

in the management address TLV.

Work mechanism

LLDP operating modes

An LLDP agent can operate in one of the following modes:

• TxRx mode—An LLDP agent in this mode can send and receive LLDPDUs.

• Tx mode—An LLDP agent in this mode can only send LLDPDUs.

• Rx mode—An LLDP agent in this mode can only receive LLDPDUs.

• Disable mode—An LLDP agent in this mode cannot send or receive LLDPDUs.

Each time the LLDP operating mode of an LLDP agent changes, its LLDP protocol state machine

re-initializes. A configurable re-initialization delay prevents frequent initializations because of frequent

changes to the operating mode. With this delay configured, before an LLDP agent can initialize LLDP, it

must wait for the specified interval after the LLDP operating mode changes.

Transmitting LLDPDUs

An LLDP agent operating in TxRx mode or Tx mode sends LLDPDUs to its directly connected devices both

periodically and when the local configuration changes. To prevent LLDPDUs from overwhelming the

network during times of frequent changes to local device information, LLDP uses the token bucket

mechanism to rate limit LLDPDUs. For more information about the token bucket mechanism, see ACL and

QoS Configuration Guide.

LLDP automatically enables the fast LLDPDU transmission mechanism in either of the following cases:

• A new neighbor is discovered. A new LLDPDU is received and carries device information new to the

local device.

• The LLDP operating mode of the LLDP agent changes from Disable or Rx to TxRx or Tx.

With this mechanism, a specific number of LLDPDUs are sent successively at a configurable fast

transmission interval to help LLDP neighbors discover the local device as soon as possible. Then, the

normal LLDPDU transmit interval resumes.

Receiving LLDPDUs

An LLDP agent that is operating in TxRx mode or Rx mode checks the validity of TLVs carried in every

received LLDPDU. If valid, the information is saved and an aging timer is set for it based on the TTL value

in the TTL TLV carried in the LLDPDU. If the TTL value is zero, the information ages out immediately.

Protocols and standards

• IEEE 802.1AB-2005, Station and Media Access Control Connectivity Discovery

• IEEE 802.1AB-2009, Station and Media Access Control Connectivity Discovery

• ANSI/TIA-1057, Link Layer Discovery Protocol for Media Endpoint Devices

• DCB Capability Exchange Protocol Specification Rev 1.00

• DCB Capability Exchange Protocol Base Specification Rev 1.01