Network Discovery

Link Layer Discovery Protocol (LLDP)

A vendor-neutral Layer 2 protocol for heterogeneous networks

Link Layer Discovery Protocol (LLDP) is a layer 2 neighbor discovery protocol that allows devices to advertise device information to their directly connected peers/neighbors. It is best practice to enable LLDP globally to standardize network topology across all devices if you have a multi-vendor network.

Commonly used layer 2 discovery protocols are often vendor-proprietary, for instance, Cisco’s CDP, Foundry’s FDP, Extreme’s EDP and Nortel’s NDP. This makes layer 2 discovery difficult in a heterogeneous environment. To counter this, IETF has introduced a standard vendor-neutral configuration exchange protocol – the LLDP.

Using LLDP, device information such as chassis identification, port ID, port description, system name and description, device capability (as router, switch, hub…), IP/MAC address, etc., are transmitted to the neighboring devices. This information is also stored in local Management Information Databases (MIBs), and can be queried with the Simple Network Management Protocol (SNMP). The LLDP-enabled devices have an LLDP agent installed in them, which sends out advertisements from all physical interfaces either periodically or as changes occur.

Network monitoring software (like WhatsUp Gold) uses SNMP queries to scan a device’s MIB to retrieve neighbor information. Plus its built-in layer 2/3 network discovery and mapping uses LLDP in conjunction with its ARP Cache Discovery method, as well as other layer 2/3 protocols, to trawl through the interconnected devices, retrieving information to build a detailed topology map. As LLDP works purely at the data link layer, WhatsUp Gold will discover devices with no recent IP traffic, as well as devices in a blocked state.

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