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New Member

CoS Tagging off an IP Phone Port

So, if you wanna mark an ethernet frame with a CoS value, you can use the header of an ISL or .1q frame to do so. A plain 802.3 or 802.3u frame doesn't have a place to mark CoS, right?

CCO says, "You can connect a PC or other data device to a Cisco 7960 IP Phone port. The PC can generate packets with an assigned CoS value." at

How can a PC mark CoS without having their NIC card set to trunking?

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New Member

Re: CoS Tagging off an IP Phone Port

CCO also says, "Layer 2 802.1Q frame headers have a 2-byte Tag Control Information field that carries the class of service (CoS) value in the three most-significant bits, which are called the User Priority bits. On interfaces configured as Layer 2 802.1Q trunks, all traffic is in 802.1Q frames except for traffic in the native VLAN. Other frame types cannot carry Layer 2 CoS values." at:

Any ideas? thanks a million.

New Member

Re: CoS Tagging off an IP Phone Port

You are correct in that CoS can only be transported when using trunking. Now :

1. Cisco IP phones support 801.1q frame on their PC port not only toward the switch.

2. The CoS value override is done at the Ethernet switch, so even if the PC traffic is not using 802.1q, ie native VLAN, CoS value will be assigned when traffics enter the switch.


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