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I've recently began working for a VoIP Provider and I am constantly seeing VoIP Phones pulling different VLANs than the Host that is connected to it i.e. daisy-chained.

When I've tried to look more into this the only method I can find is to set the VLAN Tag in the phone itself as otherwise I don't see why both wouldn't get whatever Tag is set on the Switch Port.

Is there another way to accomplish this? I understand the usefulness of the setup, I'm just not sure how it's being done and haven't been able to find anything that doesn't involve the phone itself.

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  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could post and accept your own answer.
    – Ron Maupin ♦
    Commented Jan 5, 2021 at 20:56

2 Answers 2

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A VoIP phone that chains to another device is a switch, and it negotiates a trunk between the phone and the switch. This happens with CDP or LLDP.

For example, a Cisco switch interface configured as an access interface connecting to a Cisco phone will use CDP to negotiate a trunk from the access interface:

interface GigabitEthernet 1/0/1
 switchport mode access
 switchport access vlan 10
 switchport voice vlan 20

If you connect a PC to the switch interface, you will have an access interface using VLAN 10, but if you connect a Cisco phone, you will get a trunk with both the access VLAN 10 and the VoIP VLAN 20.

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    do you know how the switch detects the Cisco phone? CDP?
    – jonathanjo
    Commented Oct 30, 2017 at 17:03
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    Yes, it uses CDP for a Cisco phone. We have had some other phones, too, where we had to enable LLDP for them to do this.
    – Ron Maupin ♦
    Commented Oct 30, 2017 at 17:04
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Certainly this is the usual situation for the Cisco and Avaya phones I'm familiar with, as it's usual to keep voice and data traffic separate.

This is from a Cisco SPA504G phone web configuration; these facilities are available through any of the methods for configuring them, through their remote provisioning mechanisms (HTTP/TFTP etc) enter image description here

You see that the phone VLAN and the PC port VLAN don't necessarily have the same value.

What make and model of phones are you working with? That would be helpful.

Jonathan.

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  • Not using a particular make or model, was just more curious if that was the way to do it from a macro pov. Seems like it is and my research was on the right track. :]
    – Josh Rowley
    Commented Oct 30, 2017 at 17:09

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