CoS question

Unanswered Question
Feb 10th, 2009


I'm studying for the BCMSN exam. I use the Official Exam Certification Guide from Cisco Press. In the Q&A Chapter 14 (IP Telephony) section I found an answer on a question that I couldn't explain for myself.

The question:

Can the CoS information from the voice traffic be passed when the “switchport voice vlan

untagged” command is used? If so, how?

The answer:

Yes, CoS information is passed within the 802.1p portion of the 802.1Q trunk


My problem with it:

My thoughts are that the voice (and data) traffic will go over the native VLAN with the configuration command shown. And that traffic over the native VLAN is untagged. And without an 802.1Q tag there couldn't be any CoS information in the frames. Where do I go wrong?

Thanks in advance!

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Overall Rating: 3 (1 ratings)
jrensink78 Wed, 02/11/2009 - 07:26

The answer would be correct if the command was "switchport voice vlan dot1p". Using the untagged option should not pass any CoS info in the traffic.

But all traffic will have the layer 3 IP precedence value set. You can see some Cisco documentation at the link below.

rvdeijk1977 Wed, 02/11/2009 - 08:43

Thank you!

I've read the document you've linked to. The document is still a bit vague about it but I conclude that even if the “switch(config-if)# switchport voice vlan untagged” command is used, the frames in the native VLAN of the special 802.1Q trunk will be tagged with 802.1p CoS information. This is similar to what the book very vaguely describes (only in 1 figure if I'm right). Of course I know that there is IP-precedence information in the ToS field of the IP packet header. But the question is purely about layer 2.

Can someone with the right knowledge please confirm or reject my conclusion. I want to learn it right!


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