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Community Member

Voice VLAN Packets

Hi Everybody,

I am trying to configure Voice VLAN. While going through the document a point raised when read the following Guidelines from Cisco.

"The Cisco IP Phone uses IEEE 802.1Q frames for Voice VLAN traffic.Voice VLAN is only supported on access ports and not on trunk ports,even though the configuration is allowed"

It seems ironical that when the device sends the CoS bit set, It should have ISL/802.1Q Frame. If the switch port is configured as Access Ports, how the device connected to the port (In this case IP Phone) is sending the CoS Bits to Switch?

Can you please help me in understanding this?

RBK

1 ACCEPTED SOLUTION

Accepted Solutions
Blue

Re: Voice VLAN Packets

Hi:

My understanding is that it depends on the way you configure the port. If it's an access port and uses a voice vlan, CDP will convey that information to the switch, as well as the CoS markings. If you have configured the port to trust CoS settings, then the switch will. If not, the switch will mark the voice packets in accordance with theQoS policy you have configured.

If the interface is configuraed as a trunk port, the CoS markings will be conveyed via the dot1q tag in the dot1q packet. As far as trusting, it is the same paradigm as the access port.

Makes sense?

Victor

4 REPLIES
Blue

Re: Voice VLAN Packets

Good afternoon:

The switch uses CDP to transfer to the phone configuration information regarding Class of Service (CoS), and the VLANs to be used for voice and data traffic.

The Extended Trust field contains a boolean value. If it is true, the port trusts the CoS (Class of Service) values in incoming packets. Otherwise, the port will assign its own CoS value to the traffic.

I do think you have to be running CDPv2.

HTH

Victor

Community Member

Re: Voice VLAN Packets

Hi Victor,

Thanks for your reply.

Shall I presume based on your answer that, In case of IP Phones, it uses CDP to communicate CoS Bits and not 802.1Q/ISL.

Did I understood in right way. Thanks in advance

RBK

Blue

Re: Voice VLAN Packets

Hi:

My understanding is that it depends on the way you configure the port. If it's an access port and uses a voice vlan, CDP will convey that information to the switch, as well as the CoS markings. If you have configured the port to trust CoS settings, then the switch will. If not, the switch will mark the voice packets in accordance with theQoS policy you have configured.

If the interface is configuraed as a trunk port, the CoS markings will be conveyed via the dot1q tag in the dot1q packet. As far as trusting, it is the same paradigm as the access port.

Makes sense?

Victor

Hall of Fame Super Gold

Re: Voice VLAN Packets

Despite the port is configured as access, like the the guideline says, the phones still use an 802.Q frames.

In any case, note that in normal LAN environments, the use of COS bit is not important and everything works even without it.

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