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. Rene,

    It’s always so good read your articles! I can understand them and make me feeling smarter :smiley: Thanks!!!

    Hug

  2. Hi Rene,

    I work as technical support in Internet Services & Communications company and we don’t use Cisco products only, we also use other network products from different vendors like Ubiquiti, mikrotik, Exalt, etc. and we connect all these different products with each other!!
    so when I use CDP to show which devices connected to cisco switch or router I noticed that information of different vendors products also appears to me!!
    My question is why I show this info ? I know that CDP is Cisco proprietary so how it’s able to identify the products from other vendors???

  3. Rene,

    Cisco ASA doesn’t seem to have CDP available. Is it maybe configured a different way?

    Chris

  4. Hello @ReneMolenaar ,

    I work with Cisco ASRs devices running cisco XR OS, and they don’t have CDP enabled by default. You have to enable it globally withcdp run command and that’s not enough to make it work. You still have to enable it per interface as well with cdp enable command.

    Regards.

  5. @chrismchavez,

    ASAs don’t have CDP feature and it make sense since they are firewall devices. :slight_smile: but there is a trick way to do this, I will lab it and show you the commands.

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