LLDP helps troubleshoot, map and more


The soon-to-be-ratified IEEE standard, 802.lAB Link Layer Discovery Protocol, promises to simplify troubleshooting of enterprise networks and enhance the ability of network management tools to discover and maintain accurate network topologies in multi-vendor environments. The protocol is expected to become an official standard next month.

LLDP is a neighbor discovery protocol. It defines a standard method for Ethernet network devices such as switches, routers and wireless LAN access points to advertise information about themselves to other nodes on the network and store the information they discover. Details such as device configuration, device capabilities and device identification can be advertised using this protocol.

In particular, LLDP defines a set of common advertisement messages, a protocol for transmitting the advertisements and a method for storing the information contained in received advertisements.

Multiple advertisement messages are transmitted in one LAN packet by the advertising device in the form of a type length value (TLV) field.

All LLDP-capable devices must support device chassis ID and port ID advertisements, but it is expected that most implementations also will support system name, system description and system capabilities advertisements. System name and system description advertisements provide useful information for collecting network inventory data. System description advertisement can contain data such as the advertising device's full name and the version identification of the system's hardware type and software operating system.

LLDP information is transmitted periodically and stored for a finite period. IEEE has defined a recommended transmission rate of 30 seconds, but the transmission rate is adjustable. LLDP devices, after receiving an LLDP message from a neighboring network device, will store the LLDP information in an IEEE-defined Simple Network Management Protocol (SNMP) Management Information Base (MIB). LLDP information is stored in the SNMP MIB and is valid for a period of time defined by the LLDP "Time to Live" (TTL) value that is contained within the received packet. IEEE recommends a TTL value of 120 seconds, but it can be set to a maximum value 65,000 seconds or a minimum value of 0 seconds. Every time a device receives an LLDP advertisement packet, it will store the information within it and initialize a timer that will be compared to the TTL value. If the timer reaches the TTL value, the LLDP device will delete the stored information. This ensures that only valid LLDP information is stored in the network devices and is available to network management systems.

The protocol lets network management systems accurately discover and model physical network topologies. As LLDP devices transmit and receive advertisements, the devices will store information they discover about their neighbors. Advertisement data such as a neighbor's management address, device type and port identification is useful for examining what devices are neighbors of each other, and through what ports they connect to each other.

The IEEE has further enhanced the value of the LLDP protocol by making it possible for other standards organizations and vendors to create custom advertisement messages. The Telecommunications Industry Association (TIA), an International Standards Organization-certified group that develops standards-governing enterprise telephony systems, is developing an extension to LLDP for use in VoIP network equipment. The TIA standard will add VoIP and telephony-specific advertisement fields that provide information about VoIP phones to the network and could aid in the development of a VoIP E911 emergency calling service standard.

LLDP will be a useful management tool - particularly for heterogeneous networks - by providing accurate network mapping, inventory data and network troubleshooting information.  It is expected that additional applications will evolve as the industry fully implements the standard.

How it works: LDP

Allen and Frattura are directors of secure networks solutions at Enterasys Networks. They can be reached at aallen@enterasys.com and Frattura@enterasys.com, respectively.

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