Link Layer Discovery Protocol (LLDP)

LLDP is a layer two discovery protocol, similar to Cisco’s CDP. The big difference between the two is that LLDP is a standard while CDP is a Cisco proprietary protocol.

Cisco devices support the IEEE 802.1ab version of LLDP. This allows non-Cisco devices to advertise information about themselves to our network devices.

LLDP uses attributes that contain a type, length and value descriptions. These are called TLVs (Type, Length, Value). Devices that support LLDP use TLVs to send and receive information to their directly connected neighbors. Here’s an example of some basic TLVs:

  • Port description TLV
  • System name TLV
  • System description TLV
  • System capabilities TLV
  • Management Address TLV

Some network end devices (like IP Phones) can use LLDP for VLAN assignment or PoE (Power over Ethernet) requirements. To accomplish this, an enhancement was made which is called MED (Media Endpoint Discovery). This is typically known as LLDP-MED.

Configuration of LLDP is really simple, depending on your switch and IOS version it might be enabled or disabled by default. Let’s take a look at an example:

sw1-sw2

I have two Cisco Catalyst 3560 switches, directly connected to each other. LLDP is disabled by default on these switches so let’s enable it:

SW1, SW2
(config)#lldp run

This enables LLDP globally on all interfaces. After a couple of seconds we can see something:

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Forum Replies

  1. Hi Lazaros,
    Would you please explain TLVs in detail?
    And how end devices (like IP phones) use LLDP for VLAN assignment or PoE requirements?

  2. Hello Muhammad

    A Type Length Value (TLV) is an encoding scheme used to add optional additional elements within a communications protocol. It is a general scheme that is used for various protocols, including LLDP. Each TLV, as its name suggests, includes three fields:

    1. Type code - this is a code that is used to specify what kind of information is included in this TLV. For example, for LLDP, a code of 2 means the TLV contains information about the Port ID.
    2. Length - This value indicates the length of the field containing the value, in bytes.
    3. Value - this field
    ... Continue reading in our forum

  3. Hello Boris

    LLDP is a protocol whose purpose is to exchange information between devices at Layer 2. As such, in order for it to function correctly, it must have bidirectional communication between devices. LLDP has no configuration parameters that would enable an interface to accept LLDP frames but not to send them.

    However, if you want to, you can use a MAC address access list and block outbound traffic to the 01:80:c2:00:00:0e MAC address which is the multicast destination MAC address used by LLDP.

    A similar functionality is available on Cisco small busines

    ... Continue reading in our forum

  4. Hello Boris

    I stand corrected. I haven’t actually used LLDP in a production environment, so I was not aware of these configuration parameters, so thank you for the heads up. It seems that LLDP can be enabled or disabled globally on a switch by issuing the lldp run command in global configuration mode. If you do this, then you don’t need to configure anything at the interface level by default, as receive, transmit, and tlv-select are all enabled by default.

    However, once globally enabled, if you choose to adjust the operation of LLDP at the interface level, y

    ... Continue reading in our forum

  5. Hello Laz,

    Great explanation! Thank you very much.

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