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What is the harm of configuring a single vlan to carry both voice and data traffic?

What is the harm of configuring a single vlan to carry both voice and data traffic?

I just took over an office that is running VOIP and the previous admin has all switchports configured as such:

interface GigabitEthernet0/40
 description DATA+VOICE
 switchport access vlan 27
 switchport mode access
 switchport voice vlan 27
 srr-queue bandwidth share 1 30 35 5
 priority-queue out 
 mls qos trust device cisco-phone
 mls qos trust cos
 auto qos voip cisco-phone 
 spanning-tree portfast

New Member

Hi, There is no harm to


There is no harm to configure as above, but it is good for you to configuring a seperate voice vlan maybe you can need to apply security, qos etc things it will ease your configuration.

Super Bronze

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Well that's an interesting port configuration!


I'm guessing all the data egressing the port is untagged and while perhaps the phone tags its packets.


Unsure there's any advantage of such a configuration vs. not defining a voice VLAN.


Some of the harm is data hosts will see VoIP broadcasts they don't need to see and the VoIP phone will need to "open" data broadcasts to determine they are needed by the phone.


In BrandX VoIP documentation, mention is made you can safely scale to twice as many ports if the data and VoIP hosts are on their own VLANs.

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