What is Link Layer Discovery Protocol (LLDP)?

By | 28th July 2016


Understanding LLDP and LLDP-MED

The smooth running operation of the various networking devices in a LAN or switched network means that all protocols and applications are enabled and that all devices and are configured correctly.
However, the larger the network gets, the more difficult it will be for the network administrator to control, manage and sort out configuration problems.

This is where the IEEE 802.1AB Link Layer Discovery Protocol (LLDP) steps in.

If your network is running only Cisco network devices (routers, bridges, access servers, and switches), is a known fact that Cisco Discovery Protocol (CDP) which runs over layer 2 (data link layer) can be used for network management of applications and to automatically discover and learn about other Cisco devices connected to the network.

The Link Layer Discovery Protocol (LLDP) is a protocol that can be used to support non-Cisco devices on your network.

LLDP is a neighbor discovery protocol that is used for network devices to advertise information about themselves to other devices on the network and learn about each other.

LLDP like CDP runs over the data-link layer of your network that includes non-Cisco devices or different network layer protocols.


How does LLDP work?

LLDP enabled network devices regularly exchange LLDP advertisements with their network neighbors and store this information in their internal database (MIB).
A Network Management Software – NMS can use SNMP to access this information to build an inventory of the network devices connected to the network and other applications.

Features of LLDP

LLDP have some features it uses in advertising, discovering and learning neighbor devices. These attributes contain type, length, and value descriptions and are referred to as TLVs.

TLVs are used by LLDP to receive, send and gather information to and from their neighbors. Details such as configuration information, device capabilities, and device identity are information advertised using this protocol.

Cisco Catalyst switch supports the following basic management TLVs:

•Port description TLV

•System name TLV

•System description TLV

•System capabilities TLV

• Management address TLV

These organizationally specific LLDP TLVs are also advertised to support LLDP-MED.

•Port VLAN ID TLV ((IEEE 802.1 organizationally specific TLVs)

•MAC/PHY configuration/status TLV(IEEE 802.3 organizationally specific TLVs)

How to configure LLDP

Disabling and Enabling LLDP on an Interface
LLDP is disabled globally on all supported interfaces. You must enable LLDP globally to allow a device to send LLDP packets. However, no changes are required at the interface level.

You can configure an individual interface to selectively not to send and receive LLDP packets with the no lldp transmit and no lldp receive commands.

This example shows how to globally enable LLDP.

Switch# configure terminal
Switch(config)# lldp run
Switch(config)# end

This example shows how to globally disable LLDP.

Switch# configure terminal
Switch(config)# no lldp run
Switch(config)# end

Understanding LLDP-MED

LLDP for Media (LLDP-MED) is an extension to LLDP that operates between endpoint devices such as IP phones and network devices such as switches.

LLDP-MED specifically supports voice over IP (VoIP) applications and provides additional TLVs for capabilities discovery, network policy, Power over Ethernet, inventory management, and location information. By default, all LLDP-MED TLVs are enabled.

Configuring LLDP-MED TLVs
By default, the Cisco catalyst switch only sends LLDP packets until it receives LLDP-MED packets from the end device.

The switch continues to send LLDP-MED packets until it only receives LLDP packets.

This example shows how to enable a TLV on an interface when it has been disabled.

Switch# configure terminal
Switch(config)# interface GigabitEthernet1/0/1
Switch(config-if)# lldp med-tlv-select inventory management
Switch(config-if)# end

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