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DEMO: LLDP

Link Layer Discovery Protocol (LLDP) is an open, vendor-neutral alternative to Cisco's proprietary CDP, enabling network device discovery across multi-vendor environments. This content covers how to enable and disable LLDP globally and at the interface level on Cisco equipment.

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About this video

Link Layer Discovery Protocol (LLDP) is a vendor-neutral, open-standard Layer 2 discovery protocol designed to work across equipment from multiple manufacturers. Cisco Discovery Protocol (CDP) is Cisco-proprietary and will not function on non-Cisco devices, making LLDP the practical choice for networks that include equipment from different vendors. On Cisco devices, LLDP is disabled by default and must be explicitly enabled through global configuration using the command lldp run. Once active, LLDP sends neighbor advertisements every 30 seconds — somewhat faster than CDP — and retains discovered neighbor entries in its table for 120 seconds after the last received update before those entries expire. LLDP also differs from CDP in how it is disabled at the interface level: rather than a single command, both the transmit and receive functions must be turned off separately using no lldp transmit and no lldp receive. This granular control allows administrators to keep LLDP running globally while preventing specific interfaces from participating in discovery. Reversing that configuration is equally straightforward, requiring only that the transmit and receive commands be re-entered on the target interface. LLDP can be shut down entirely across the device by issuing no lldp run in global configuration mode.

What you'll learn

What's covered

LLDP on Cisco Devices

Aligned to

Cisco CCNA
2.3 Configure and verify Layer 2 discovery protocols (CDP and LLDP)

Key terms

Cisco Discovery Protocol
CDP
A Cisco proprietary Layer 2 protocol that allows Cisco devices to discover and share information about directly connected Cisco neighbors, including device type, IP address, and interface details. CDP operates independently of Layer 3 protocols and is used for network topology mapping and troubleshooting.
Link Layer Discovery Protocol
LLDP
An IEEE 802.1AB vendor-neutral protocol that allows network devices to advertise their identity, capabilities, and directly connected neighbors to other devices on the same LAN. LLDP is the open-standard equivalent of Cisco's CDP and enables multi-vendor network topology discovery.
Data Link Layer
Layer 2 of the OSI model responsible for node-to-node data transfer and error detection on a single network segment.
Protocol
A set of rules governing the format and transmission of data between devices.
Advertisement Timer
The interval at which a discovery protocol sends out neighbor information; LLDP advertises every 30 seconds by default on Cisco devices.
Hold Timer
The duration a discovery protocol retains a neighbor entry in its table without receiving a new advertisement; LLDP holds entries for 120 seconds by default.

Topics

Lldp Cdp Network Device Discovery Cisco Ios Interface Configuration Networking

Transcript

Why We Need LLDP

CDP is something specific to Cisco equipment. It is a Cisco proprietary protocol, so you won't see it on other devices that are out there. So what happens if you have other different brands of equipment that are connected to your network and you want some sort of discovery protocol running on it? Well, there is an answer to that, and the answer is Link Layer Discovery Protocol. Your Cisco equipment doesn't have it turned on by default, but we can turn it on in case we have other brands that are connected to your Cisco equipment.

Turning LLDP On

So let's go ahead and turn it on. The first thing that I'm going to do is I'm going to do a show lldp, and we can see that it's not enabled. So now I'm going to get into the global configuration mode to enable it, and the command to enable it is lldp run. And so now I've enabled it. I'm going to exit out of this and do a show lldp neighbors, and they've already reported in, which was pretty fast. We know CDP sometimes it took a little bit.

So let's go ahead and do a show lldp, and that's going to show us some information. These advertisements are actually sent every 30 seconds, so it's sending out the advertisements a little bit faster, and then it will hold on to entries. It will stay in this table right here for 120 seconds even if it doesn't receive an update for 120 seconds, and then it will expire out of there. So it updates a little bit faster than CDP does.

Turning It Off on a Single Interface

Now let's say I want lldp to be running, but I don't want it to be communicating to this S3 connection right here. So what I can do is I can get into that interface, and I'm going to get into 1 0 45, and I'm going to say no lldp. Now there is a little bit of a difference between lldp and CDP, in that we have to turn it off for both the transmit and the receive. So I'm going to do a no lldp receive and no lldp transmit.

Now it's still going to be in the table at this point in time, so I'm going to do show lldp neighbors and hit enter. It's still in here because it's going to hold it for 120 seconds. So once it doesn't receive an update, an advertisement, for 120 seconds, then it'll come out of this.

So you probably can guess how we turn it back on. We'll just do an update here. I'm not sure if we've given it enough time, but to turn this back on, all we have to do is re-enter this command, lldp receive, and it will turn it back on, and then we can do the lldp transmit. So I've entered this into the interface, and now that interface is back on.

Turning LLDP Off Altogether

I'm going to exit out of here, and if I want to turn lldp off altogether I can do no lldp run and hit enter, and now lldp is turned off. Just to show, I'm going to do a show lldp, and we see that it is not enabled. So there you have it, another alternative to CDP. You have to turn it on.

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