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 (if LLDP is received by the switch). 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>enable
Switch#show
running-config | include lldp
(use this to verify there
are no lldp entries)
Switch#config term
Switch(config)#lldp
run
Switch(config)#Ctrl-z
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 LLDP 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.
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'NetScanTools Pro', 'NetScanTools Standard', 'NetScanTools LE', 'ipPulse', 'Northwest Performance Software' and 'NetScanTools.com', are trademarks of Northwest Performance Software, Inc.
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