ip phone L2 cos marking

Unanswered Question
Mar 3rd, 2009

Hi all,

I have read lots of documents about how ip phone mark parckets in the 802.1p field in the 802.1q header which results in having more questions.

From what i understood, when a cisco ip phone is connected to a cisco switch, the switch sends CDP messages to the ip phone to tell it the way it should mark packet. Clearly, it is supposed to be voice traffic (cos=5), signaling (cos=3), pc data traffic (cos=0)

Is it correct?

If yes, then i am wondering how the phone mark packets when it is connected to a non-cisco switch?

Clearly the non-cisco switch will not send CDP messages to the cisco ip phone.

How does the phone mark voice, signaling and data traffic? Does it marks it at all?

Thanks in advance

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zanees Tue, 03/03/2009 - 13:46

Another way marking is done is by using access lists. If you have option of using access list then mark traffic using specific port number like 2000 and 2002 tcp ports for signaling and standard RTP UDP ports for media.

julien.krieger@... Wed, 03/04/2009 - 01:42

You are talking about layer 3 marking. I want to make sure my ip phones are marking at Layer 2?

Problem is that i can't see the 802.1Q tags with wireshark. So i can't see exactely what the phones do at L2.

My client has non-cisco switches and my phones are connected onto these switches. They can only deal with 802.1q cos marking.

I have the feeling the phones do not mark the packets at the L2 layer.

harris.h Wed, 03/04/2009 - 11:40

CoS is carried in the 3 bit priority bit inside the frame header, The default as you said is cos 5 for voice traffic and 3 for signaling, I guess this will be marked as such when leaving the phone.

Im sure I have seen a setting to change the 5 and 3 defaults for the phones.

How are you extending Vlans to the phones? ie. VVlan

julien.krieger@... Wed, 03/04/2009 - 13:14

The phones are connected to a non-cisco switch, therefore phones are conencted to the non-cisco switch through a trunk.

The voice vlan id is entered manually on the phone.

I have tried to find where to change the cos values but i could not.

Are the cos marking values coded in the firmware? or are they learned by the phone from the conf it downloads from the CUCM?


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