LLDP and VOIP

[dcanady55]

Hello,

Environment...Mitel 3300 PBX, Catalyst 2960 switches. 1 data vlan and 1 voice vlan.

I am trying to understand the phones boot process and how LLDP plays a role in this process. With LLDP turned on when the phone boots up it sends the switch a plethora of information but what information exactly does it say hey I'm a VOIP phone place me in the voice vlan please? DHCP is setup on the PBX and I don't have it handing out the voice vlan so I know the switch is putting the phones in the voice vlan just not sure what piece of information makes this possible.

Ultimately, I'm interested because I've noticed when troubleshooting I will see the phones mac address registered under the data vlan and voice vlan and I'm trying to figure out how and if it really even matters?

Thanks for any help!

[keebs1077]

Hi, Not sure if my environment is totally the same but I have something very similar with what may be a good explanation. We have all Cisco switches and a Shoretel voip system which includes Shoregear switches and Shoretel phones. I set up the voice as well as configurations for the DHCP server on our network. When the Shoretel phone first boots, it starts off on the data Vlan and then re-configures itself and registers with the Shoretel server. After that happens, the phone registers with the Shoretel server and hops on the voice Vlan. LLDP is a standard IEEE protocol so that devices can discover devices from different manufacturers. .

[dcanady55]

Thanks for the response. That doesn't clear it up for me though. I know what LLDP is but still not sure how it helps the device obtain access to the voice vlan. I did look through the phones configuration and it shows LLDP turned on which makes sense so I assume there's some kind of packet it broadcasts to the switch which indicates it's Voice and the switch then puts this on the correct VLAN.

[keebs1077]

Wireshark will probably show the contents of the exchange between the phone and switch. There is likely some flag set to on in the packet for voice or some other value so the switch knows the device connected should be on the voice vlan.

[sleb23]

LLDP-MED (Media Endpoint Devices)

Part of the exchange is the 'Capabilities Discovery' - allows endpoints to determine the type of capabilities the connected device supports (phone, switch, repeater, WAP, etc.)

I don't know what the actual adjustment to the frame looks like, but I believe that's the concept behind it.

[Joseph W. Doherty]

Unsure LLDP helps a VoIP get to the voice VLAN, although I believe it helps with power management.

CDP, on the other hand, I believe can interoperate with a switch's "trust" setting to insure only a VoIP phone can take advantage of high-priority DSCP markings (e.g. DSCP EF).

As others have noted, I've seen brand X VoIP phones jump from the data VLAN to the voice VLAN, but not using CDP or LLDP. The ones I've used did this based on a DHCP option string passed to the VoIP phone when it obtained a data VLAN IP. The option string told the phone to move to a specific tagged VLAN.