Showing results for 
Search instead for 
Did you mean: 
Join Customer Connection to register!
Frequent Contributor

voip phones and tagging

Hi all, further to my questions, can anyone one tell me if put my phone say on vlan 10 and my pc in vlan 5, the pc plugs into the phone and then phone to switch, in this scenario are both pc and phone tagged ? and when we say tag, does that mean either cos value is set OR vlan, if either field is populated in the frame does the switch see that as a tagged packet ?

Also if i have say my pc traffic untagged on the port, if I change the native vlan for my trunk ports to the same either end, will the initial traffic from my pc get tagged along the trunks ?

Not applicable

You define two vlan one for voice and data . apply the exteneded QOS to the switch where the ip phone is connected and PC connected. the phone will rewrite the cos to 0 for the packets coming from the pc and the switch will not rewrite these values.

to be more clear for OP, i will write one example here


description User Phone & PC Xrd floor

switchport access vlan 10

switchport mode access

switchport voice vlan 5


best regards,



In this scenario, Lets not confuse VLAN tagging with COS tagging.

VLAN tagging works the same way as replied in your earlier post.

Lets talk about QoS (typical enterprise network):

Ip phone marks voice trafic with COS 5, IP Precedence 5 and DSCP EF.

IP phone marks signalling traffic as COS 3, Precedence 3 & DSCP AF31.

If the PC connected to phone marks the pkts with any COS value, then IP phone rewrite the PC traffic with COS 0, Precedence 0 & DSCP default.

The traffic is now passed to switch. If the switch is L2, it forwards the traffic based on COS values. But if it is L3, then Precedence or DSCP values are considered.

Since COS values are used only within a LAN, when the pkts reach the router for outside networks they have to be marked again with Precedence or DSCP, based on the incoming COS values.

hope this clarified the doubt.

pls rate if helped.

Sorry, forgot 1 thing.

You need to add "mls qos trust" on switchports directly connected to Ip phone. This will make switch retain the COS, Prec & DSCP markings. Otherwise the markings will be removed by switch.

what do we do if we do not trust the phone and what to apply cos value packets ? what would you normally do this by, vlan ?