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New Member

COS in Voice and Data VLAN??

I have a query regarding data/voice vlans and COS. My understanding is that when you have a config such as the one below, the voice vlan (20) will be trunked with a COS value in the dot1p part of the dot1q tag, and that the pc (which is directly plugged into the phone) will have its traffic sent to the switch (via the phone) untagged (actually in vlan 10) - like a native VLAN.

fastethernet 0/1

switchport access vlan 10

swithport mode access

switchport voice vlan 20

If this is true, what is the point of the command:

switchport priority extend {cos 'value' | trust}

This tells the phone whether to trust the COS settings that are sent from the PC directly attached, or to override them. But the thing that is confusing me is that the PC attached to the phone sends its traffic untagged! And the phone sends the PC data untagged to the switch. If the frames are sent untagged then there is no tag, and no where to put the COS values (such as in a dot1p field). So how can the phone trust or change the COS settings if they don't exist?

Am I making sense or is this a drastic oversight on my part, apologies if it is the latter!!

Best regards

Dazzler

1 ACCEPTED SOLUTION

Accepted Solutions
Silver

Re: COS in Voice and Data VLAN??

No. The phone can handle both tagged and untagged frames from the PC. If the PC sends a tagged frame, then the phone will pass on the tagged frame to the switch.

Regards,

Anup

4 REPLIES
Silver

Re: COS in Voice and Data VLAN??

You are right.

The switchport priority extend command doesn't have much significance in most practical applications because most PC's are not doing any tagging.

But there are NIC's that can do dot1q tagging. Mostly they are used in servers. So if you happen to have a PC that is doing tagging, then by using this command you can either override or trust those values.

Regards,

Anup

New Member

Re: COS in Voice and Data VLAN??

I understand, but the phone sends the PC data to the switch untagged. Therefore, even if the PC does send a tagged frame to the phone, the phone is expecting an untagged frame.

This is very confusing!!

Thanks for the response though :-)

Silver

Re: COS in Voice and Data VLAN??

No. The phone can handle both tagged and untagged frames from the PC. If the PC sends a tagged frame, then the phone will pass on the tagged frame to the switch.

Regards,

Anup

New Member

Re: COS in Voice and Data VLAN??

Oh right, it all makes sense now!! Many thanks for clearing this up.

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