Untagged port connecting to PC with COS field

Unanswered Question
Aug 5th, 2008

Hi, all

I am facing with a little curious question in terms of COS on untagged port.

My question is that how COS field can be marked in untagged port if i command "switchport port extend cos 1"

generally we can control COS field in accordance with tagged port as known VLAN. but For PC, it works on unttaged port.

There is no COS field in untagged packet. does anybody know about that ?

The following configuration are what I wanna do;

1. VLAN 12 for PC, Data traffic

2. VLAN 200 for IP phone, voice taffic (COS 3 : voice control, COS 5 : voice packet on default behavior)

3. Needs to mark COS 1 for data taffice

Switch --------- IP phone -------- PC

## configuration ##

switchport mode access vlan 12

switchport port extend cos 1

switchport voice vlan 200

mls qos cos trust

I have this problem too.
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Marwan ALshawi Wed, 08/06/2008 - 01:28

Multi-VLAN access ports are supported by all Cisco Catalyst switches. All data devices

typically reside on data VLANs in the traditional switched scenario. A separate voice VLAN

may be needed when combining the voice network into the data network. Cisco Catalyst

switches using Catalyst Oerating System refer to the voice VLAN as the auxiliary VLAN

You can implement multiple VLANs on the same port by configuring an access port. A tagging

mechanism distinguishes among VLANs on the same port. 802.1Q is the IEEE standard for

tagging frames with a VLAN ID number. The IP phone sends tagged 802.1Q frames. The PC

sends untagged frames and the switch puts the frame into the configured access VLAN. When

the switch receives a frame from the network destined for the PC, it removes the access VLAN

tag before forwarding the untagged frame to the PC

i think this information has answered your question

good luck

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jsnoh0228 Wed, 08/06/2008 - 04:31

Thanks a lot, marwanshawi,

I meant COS field. If you mark COS 5 for voice,and COS 2 for Data, how does switch fill cos 2 in untagged packet ?

It makes sense Voice packet, because Voice will be tagged with VLAN ID. but For data traffice,PC, there is no cos field, because Data traffice will be unttagged.

could you explain about that ?

Marwan ALshawi Wed, 08/06/2008 - 05:00

In order to avoid this, remove the trunk configuration and keep the voice and access VLAN configured along

with Quality of Service (QoS). Technically, it is still a trunk, but it is called a Multi-VLAN Access Port

(MVAP). Because voice and data traffic can travel through the same port, you should specify a different

VLAN for each type of traffic. You can configure a switch port to forward voice and data traffic on different

VLANs. Configure IP phone ports with a voice VLAN configuration. This configuration creates a pseudo

trunk, but does not require you to manually prune the unnecessary VLANs

because Technically, it is still a trunk

it can have cos field

but it is called a Multi-VLAN Access Port

i think now more clear :)

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