IEEE 802.1AB Link Layer Discovery Protocol (LLDP) simplifies troubleshooting of enterprise networks and enhances the ability of network management tools to discover and maintain accurate network topologies in multi-vendor environments. It defines a set of advertisement messages, called TLVs, a protocol for transmitting and receiving the advertisements, and a method for storing the information contained in the received advertisements.

The LLDP protocol enables stations attached to a LAN to advertise information about the system (such as, its major capabilities and its management address) and information regarding the station’s point of attachment to the LAN (port ID and VLAN information) to other stations attached to the same LAN. These can all be reported to management stations through IEEE-defined SNMP MIBs.

LLDP information is transmitted periodically. The IEEE has defined a recommended transmission rate of 30 seconds, but the transmission rate is adjustable. An LLDP device, after receiving an LLDP message from a neighboring network device, stores the LLDP information in an SNMP MIB. This information is valid for a finite period of time after TLV reception. This time is defined by the LLDP Time to Live (TTL) TLV value that is contained within the received packet unless refreshed by a newly received TLV. The IEEE recommends a TTL value of 120 seconds, but you can change it if necessary. This ensures that valid LLDP information is stored in the network devices and is available to network management systems.

LLDP information is associated with the specific device that sends it. The device itself is uniquely identified by the receiving party port through chassis ID and port ID values. Multiple LLDP devices can reside on a single port, using a hub for example, and all the devices are reported through MIB. You can enable (Rx-only, TX-only, and Rx or Tx) or disable LLDP mode of operation on a per-port basis.