• A Network Switch Mapping Solution

What is LLDP?

LLDP (Link Layer Discovery Protocol) is a vendor independent IEEE standard method of advertising information about a device through a network connection to another device. It can assist in determining network topology. The LLDP information is sent from device interfaces at a fixed interval and when received by a switch, the LLDP data is recorded by the switch. The data can consist of as little as the sending interface MAC address (Windows 10/8.x) to details about the system name, description, IP address and capabilities.

The LLDP information presented by the Switch Port Mapper
When the LLDP column is visible, the LLDP cells show information from the device directly connected to the port. This may include the sender's MAC address, IP address, port name, capabilities, port description and system name. Any information not sent is marked 'not advertised'. If no LLDP is recorded for that port by the switch, you will see N/A in the cell.

How does LLDP relate to Network Topology?

LLDP can help identify the device physically connected to the switch port. By carefully examining the LLDP information the switch receives, you can build a network topology map. It is important that as many devices as possible have LLDP enabled and your switch also have LLDP enabled. LLDP is enabled by default in Windows 10 and 8.x, however, LLDP is not always enabled. Most devices like switches, routers, VOIP phones have LLDP capabilities. You may need to turn LLDP on in the device settings and most importantly in the switch.

When does having LLDP information matter?
Any time there is more than one MAC address appearing attached to a switch port LLDP can help you decide which MAC address is the actual attached device.

Example scenarios where multiple MAC addresses will appear:
Switch attached to another switch.
Switch attached to a server running virtual machines.

How to turn on LLDP in the switch

In order to report LLDP packets received from devices attached switch ports, you will need to enable LLDP in the switch. Often this is done from the switch web interface - be sure to write/save any changes you make. But it also may be done from the CLI.

Cisco Switches running IOS - add this to your running-config:
Switch#show running-config | include lldp (use this to verify there are no lldp entries)
Switch#config term
Switch(config)#lldp run
Switch#copy running-config startup-config (this copies the current running configuration so that when it's rebooted it comes back)

HP/HPE Switches - LLDP is enabled globally by default. If it is not enabled, this command starts it globally. 'lldp enable' can be used to control operation on a per-port basis.
HP Switch(config)# lldp run

Dell Force10 Switches - add this to your running-config and save it to your startup-config.
#protocol lldp
#advertise management-tlv system-capabilities
#no disable.

Free, open source LLDP service for Windows

Many versions of Windows do not natively transmitt LLDP packets making it hard to discover them but there is a free simple solution. WinLLDPService installs with minimal effort on Windows and sends LLDP packets. It does require WinPcap to transmit the LLDP packets. If you have Wireshark or NetScanTools Pro installed, you already have WinPcap installed. We use this software on a Windows Server 2012 machine.

You can download WinLLDPService here. Operational note: if you are using VirtualBox or similar and you start a new VM, you may need to restart the WinLLDPService service.