voice vlans on switches

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May 13th, 2009
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Hi all

with voice vlan config on my switch, what does the command do, does it tag packets with with whatever vlan comes after the command, and then reads untaged packets for the normal access vlan?

so we could use a trunk with the native vlan also for this?

is that all the command does ?

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bmcginn Wed, 05/13/2009 - 06:19
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Hi there,

The switchport voice vlan command initiates CDP packets which tell the phone to send voice traffic with the correct QoS markings (both layer 2 and 3). It should be there to make sure the phones to talk on the right vlan. Also, because its a switchport )not a trunkport) and if you're using stp or pvst then portfast is automatically input onto that port.

This document may help with understanding.


carl_townshend Thu, 05/14/2009 - 06:39
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Hi there

so to clarify, the command is used to put any 802.1p or 802.1q tagged frame coming in on that port onto the voice vlan, as normally pc traffic is untagged?

if so that answers my question and I will close


Not entirely, it does not look for frames with a COS marking in them, as you could apply QoS markings to normal PC data traffic. The CoS/IPP/DSCP markings are sent to a Cisco VoIP Phone if one is connected.

The command allows a non-cisco device to send VoIP frames already tagged with the correct 802.1q value, to be switched in the Voice VLAN.

carl_townshend Thu, 05/14/2009 - 07:40
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hi there

so what if a cisco phone is plugged in?

would i still use the command? I thought any frame marked with a cos value would be classed as a tagged frame? as the .1p sits inside the .1q frame ?

That has nothing to do with it, the physical port is a swicth port, NOT a trunk port.

1) The command tells the switch to "send" the QoS values/VLAN infromation of the voice VLAN to a cisco phone when connected and detected by the switch and the command is configured under the switch port.

2) The command is also used in the cases where the VoIP phones attached is NOT a cisco phone, but you want the switch to understand any frames that have a marking in the priority field in the frame with an inserted 802.1q field, tagged with the voice vlan ID, and should be switched into the voice vlan as the switch port is a normal switch and NOT a trunk.


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