Question about switchport voice vlan command

Ok...Reading through both the previous BCMSN Exam cert guide, and it's near duplicate SWITCH Exam cert guide, there is a conflict between the information provided for the "switchport voice vlan untagged" command.  Table 14-3 states that CoS bits are NOT set on the voice packets.  But Figure 14-2, which displays each trunking mode with this command, shows that CoS bits ARE set.  I've looked around the DocCD, can't find anything definitive.  I also have the SWITCH Foundation guide, but it only speaks to the "switchport voice vlan vlan-id" command, not the untagged.

So....with that said, does "switchport voice vlan untagged" actually set CoS bits or not?  I'd like to know, so I can fix my book, because otherwise it's a great picture of what's going on.  Thanks in advance.

Jalon

Comments

  • Awesome query - I should have some time soon to research and post my finding here.

  • No, there are no pbits because there is no 802.1q tag. Which is why its "untagged". The picture has to be wrong.

  • Yes indeed - the graphic in the course is incorrect. It was fun to read about the switchport voice vlan command as a refresher in the Command Reference for a 3560. Here are the usage guidelines quoted below - a lot of great information:

    "

    You should configure voice VLAN on Layer 2 access ports.

    You must enable Cisco Discovery Protocol (CDP) on the switchport connected to the Cisco IP phone for the switch to send configuration information to the phone. CDP is enabled by default globally and on the interface.

    Before you enable voice VLAN, we recommend that you enable quality of service (QoS) on the switch by entering the mls qos global configuration command and configure the port trust state to trust by entering the mls qos trust cos interface configuration command.

    When you enter a VLAN ID, the IP phone forwards voice traffic in IEEE 802.1Q frames, tagged with the specified VLAN ID. The switch puts IEEE 802.1Q voice traffic in the voice VLAN.

    When you select dot1q, none, or untagged, the switch puts the indicated voice traffic in the access VLAN.

    In all configurations, the voice traffic carries a Layer 2 IP precedence value. The default is 5 for voice traffic.

    When you enable port security on an interface that is also configured with a voice VLAN, set the maximum allowed secure addresses on the port to two. When the port is connected to a Cisco IP phone, the IP phone requires one MAC address. The Cisco IP phone address is learned on the voice VLAN, but is not learned on the access VLAN. If you connect a single PC to the Cisco IP phone, no additional MAC addresses are required. If you connect more than one PC to the Cisco IP phone, you must configure enough secure addresses to allow one for each PC and one for the Cisco IP phone.

    If any type of port security is enabled on the access VLAN, dynamic port security is automatically enabled on the voice VLAN.

    You cannot configure static secure MAC addresses in the voice VLAN.

    A voice-VLAN port cannot be a private-VLAN port.

    The Port Fast feature is automatically enabled when voice VLAN is configured. When you disable voice VLAN, the Port Fast feature is not automatically disabled. "

  • To all who responded, I appreciate it.  I was leaning towards that way; however, I wanted to make sure.  Thanks for the detailed info.

    Jalon

  • Here is some very good explanation on voice vlan

     

    http://cciepursuit.wordpress.com/?s=voice+vlan

     

     

    HTH...

    Deepak Arora

    http://deepakarora1984.blogspot.com/

  • Very informative information.  Thank you!

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