Cisco Voice VLAN & Non-Cisco IP Phones

Unanswered Question
Mar 11th, 2007

Have a set of non-cisco ip phones (dont do CDP). However they do VLAN and CoS. I have set VLAN on the phone to 100 and CoS 5.

I have a series of Cisco 2950 and Catalyst 500. I have configured in both of them Voice VLAN to 100. Configured the ports on the switches as trunk. In the 2950 series, there is the option to trust the CoS of IP phone but not in catalyst 500.

What I would like to know since both VLAN IDs coincide (although the phone doesnt do CDP), will the Cisco switch:

1)understand that the VLAN coming in from the IP phone is actually a Voice VLAN and treat it as such

2) secondly will it trust the Cos from the phone?


I have this problem too.
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Paolo Bevilacqua Sun, 03/11/2007 - 09:41

Yes, both things are possible by means of static configuration. Also consider:

- If the phone is not daisy-chained to a PC, you will use access mode on the switch port.

- in a all 100-mbs network, it's quite possible that even if you don't configure theoretically perfect Cos/QoS, things will work OK anyway.

rsabapathee Sun, 03/11/2007 - 10:59

in fact PCs are connected to ip phones. That is why all the cisco ports are set as trunk.

i just want to clarify whether the ip phones when set to vlan 100, the cisco switches are smart enough to understand that since the VLAN ID from the ip phone coincides with its Voice VLAN, thus trating the traffic as voice

Paolo Bevilacqua Mon, 03/12/2007 - 00:16


in short - yes.

Now let's look for a moment to what means "treating traffic as voice" for a switch, in practice, prioritizing traffic on egress ports.

Unless severely congested, a port will have at most one packet waiting to be transmitted at any given time. Let's suppose two new packets come at the very exact time to be transmitted on this port, there are three cases:

- both packets are data - hence voice is not affected.

- both new packets are voice, no priority can be indicated.

- one packet is voice and other (worst case) is a large data packet. You want the voice to be transmitted first, but even if that doesn't happen, voice will be delayed by:

100,000,000 (FE speed) / 1524 (max eth len)* 8 (bits in a byte) / 1 = 0.12 mS.

That is a negligible delay.

Summary - in a switched fast ethernet environment, things will work fine even without CoS/QoS.

gajolly Mon, 03/12/2007 - 00:22


as mentioned in previous post there r 2 ways :

a) Static Configuration

b) Enable LLDP on Switch provided IP phone should support LLDP..

gajolly Mon, 03/12/2007 - 03:05

aurite thanks for an update..

i believe in that case Static Config. wil be a gud option.

rsabapathee Mon, 03/12/2007 - 10:51


It means the voice vlan in the cisco switch has no effect on the vlan configured in the ip phone.

and better wait for cisco to support LLDP to benefit of the features of Cisco Voice VLAN

Paolo Bevilacqua Mon, 03/12/2007 - 11:00

No, I don't agree.

1 - the switch wil treat properly the voice vlan by static configation, beside I've xplained above why you should nto worry too much about QoS.

2 - LLDP that is still far away to come, has still to demonstrate advantages and interoprability.

gajolly Mon, 03/12/2007 - 21:29


i agree with previous post

- Switch will definately treat properly the voice vlan by static configuration....

rsabapathee Tue, 03/13/2007 - 09:08

What are you guys refering to as static configuration?

Recall, PCs are connected to eth2 of IP Phone on native vlan. IP phone on VLAN 100. PCs get the IP from DHCP server.

Also, switch ports are configured as dot1q trunk.


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