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Tech Talk Live Blog

Link Layer Discovery Protocol and Hostnames

Roy Hoover


Link Layer Discovery Protocol (LLDP) is a protocol designed to be used by network devices for advertising their identity, capabilities, and neighbors. This is an open standard version of vendor specific protocols like Cisco Discovery Protocol (CDP).  These protocols are very useful when deploying voice over internet protocol (VOIP) telephones.

Typically, a network that includes VOIP telephones will use one VLAN (virtual local area network) for voice traffic and another VLAN for data traffic. It may be convenient to be able to configure all the ports so that a computer or a telephone can be connected to it, and the device will be placed in the correct VLAN without needing to reconfigure the switch. In some cases the telephone may have an Ethernet pass-through port to allow a telephone and PC to share a single network port. In this case the telephone needs to trunk the data VLAN through to the pass-through port where the PC is connected. The port from the wall jack to the telephone will be a VLAN trunk and contain both the data VLAN and voice VLAN.

Any of these configurations can be made manually of course. That takes lots of time and means that any future moves, adds, or changes require configuration changes on the network switch(es) and telephones. That may be a requirement for security reasons, but for busy staff looking to deploy services at scale with minimal staff we need a better way.

LLDP is the answer to this for many of us. With LLDP the switch ports are configured with the appropriate data and voice VLANs. When a telephone is connected to a port configured that way, the switch says, “I see you are a telephone, you should use VLAN X (the voice VLAN) for your communication and use VLAN Y (the data VLAN) on your pass-through port.” Any non-telephone device that is connected to that port uses the data VLAN.

I recently installed a telephone system where LLDP was not behaving properly and the telephones were using the data VLAN instead of the voice VLAN. A quick Google search found a number of articles similar to this one that say that the hostname of the switch is an optional parameter that gets passed via LLDP but in reality if you do not set a hostname for the switch it will not work.

That particular article was referencing Polycom SoundPoint telephones and a Dell switch. I was using Polycom VVX phones and a Brocade switch, and the hostname on the switch I was using WAS configured. The various search terms I was using kept returning results referring to the hostname. On a hunch I shortened the hostname to “Roy” and suddenly the phones started getting the voice VLAN information. Further testing showed that if the name of the switch was longer than 15 characters something was happening to the LLDP information and causing the phones to place themselves on the data VLAN.

So in the end the solution was very simple, use switch names that are no longer than 15 characters. Connecting all the dots between the symptom of “telephones connect on the data VLAN and not the voice VLAN” to the solution of “the switch name is too long” was a real treat. Hopefully my efforts on this will save you from the same fate.

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