802.1p when using untagged frames?

littlegrave · ‎07-06-2007

Hello,

My question is kind of silly but I feel lost. I am reading the BCMSN self-study book and right now I am at the special-case 802.1Q trunk used between Cisco IP phones and catalysts. There's a statement "if you use the command switchport voice vlan untagged, you can still carry CoS information over the trunk". Well, I don't see how this would work since all data and voice traffic is carried over the native VLAN and is untagged. 802.1p is located in the different trunk encapsulation methods and if data and voice are sent over the native vlan, how is this going to work? Am I missing something?

Thank you!

bill.efthimiou · ‎07-06-2007

I see your point and a very good one at that. The only thing that I would say is that they may be refering to layer 3 QOS. The config guide states the following. Pay attention to the last sentence:

"You can configure a port connected to the Cisco IP Phone to send CDP packets to the phone to configure the way in which the phone sends voice traffic. The phone can carry voice traffic in IEEE 802.1Q frames for a specified voice VLAN with a Layer 2 CoS value. It can use IEEE 802.1p priority tagging to give voice traffic a higher priority and forward all voice traffic through the native (access) VLAN. The Cisco IP Phone can also send untagged voice traffic or use its own configuration to send voice traffic in the access VLAN. In all configurations, the voice traffic carries a Layer 3 IP precedence value (the default is 5)."

The link:

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat3560/12225see/scg/swvoip.htm

In which case you may need to trust DSCP, rather than COS.

I can't think of anything else.

I hope this helps.

littlegrave · ‎07-06-2007

Thanks for the reply!

I am not sure if they meant layer 3 QoS though because 802.1p is defined as a QoS mechanism at the MAC level. I guess the writers most probably made a mistake.

Jon Marshall · ‎07-06-2007

Hi

I don't think they are talking about layer 3 QOS at all. As far as i know you are totally right in that the 802.1P (Cos) information is contained within the 802.1Q tag.

So i had a look at some of the switch documenation and am still none the wiser. The doc's talk about 802.1Q frames and 802.1P frames as though they were independent of each other and there is certainly the suggestion in most of the docs that the phone can send a packet with 802.1P priority on the native vlan ie. untagged vlan.

So i am as confused as you at the moment. Can anybody else shed some light on this.

Jon

steve.b · ‎02-06-2017

8 years on and the CCNP Swithch study guide (300-115) dated 2014 has the same confusing description and diagram (p114).

It uses the term 802.1p in reference to untagged native VLAN frames. In fact 802.1p is dependent on 802.1Q frame tagging.

Cisco devices regard 802.1Q frames with VLAN=0 as non VLAN ID tagged frames; their purpose is to carry just the priority information. This is the case when using:

switchport voice vlan dot1p

My source: http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/atm/configuration/15-mt/atm-15-mt-book/atm-vlan-prty-tag.pdf

When switchport voice vlan untagged is used, there is no 802.1Q VLAN tag and therefore no 802.1p field. The documentation is wrong to say "CoS in 802.1p Bits" in relation to this mode. The traffic class (DSCP) can only be available in the IP header.