What is LLDP?

The first time I heard about LLDP was from a customer who (of course) said we needed this yesterday. First it sounded to me like it would be coming from the legal department, I guess I must have mixed it up with LLM or so. After using the search-engine-of-my-choice, I found the real meaning of the word: “Link Layer Discovery Protocol”. Still a little bit difficult to say what that exactly means, I dig deeper and came up with my own personal formula: It is DHCP for the VLAN! Okay, that makes sense!

We always had the problem how we should provision the VLAN. A DHCP option is too late (as some of our competitors did), because at this point the phone is already in a specific VLAN. CDP, the protocol from Cisco, was pretty proprietary (guess what the C stands for), so it was also not the best choice for an independent vendor who wanted to get this problem solved once and forever. I heard that even Cisco dropped CDP and decided to support LLDP instead. So LLDP would be our ticket to complete plug and play, including the VLAN, the associated priority bits (QoS in the LAN) and made it possible to have VoIP in the office with a quality that is as good or better as the good old ISDN! Even if the LAN is totally overloaded with messages from some freaked out devices (as long as they are freaking out in a different VLAN), the voice VLAN would continue operatios as if nothing has happened. Imagine someone on the phone screaming to the admin: "Our network is down! Do something!" And he says, "well you are talking over the network right now."

Digging deeper, LLDP is really primitive. There are just messages being sent every minute or so, there is no handshake, response, or anything that makes life complicated. From an engineering standpoint, that’s what you want to hear! After the switch realizes that the Ethernet link has come up and the connected device starts sending LLDP frames, it starts sending a few messages as well, so that the device has the chance to figure out what VLAN it should use. Coming from the SIP world, it sounded too good to be true!

Technically speaking, the phones don’t use LLDP; they are using LLDP-MED, which is an extension specifically for desktop phones. I think the MED must be standing for “Media Device”. But essentially it is just an extension of the LLDP core protocol. A few years ago when we started the topic, it was very difficult to get Ethernet switches that supported that and it was even more difficult to configure it. But I believe in the meantime you can even configure LLDP-MED in a convenient way through the web interface.

LLDP has some more advantages for IP phones like the snom m9. It also helps you to keep track what is connected where. Because the device reports its name, its firmware version, IP address, and an “asset ID”, the administrator can easily see through the management tools for the network, where the network devices are and to what port they are connected. The “asset ID” is actually a setting that you can set from the web interface, and it is very useful to indicate where the base it (e.g. “Room 4.1.2” or “2nd floor white cabinet”).

LLDP is actually related to 802.1X, which goes hand in hand with LLDP. But let me talk about this tomorrow. To be continued!