COS does exist in the 1q header in the 1p field, however the pc nic (most) has the ability to add this to the frame. (should see "qos packet scheduler" under you nic properties in windows) The 1q header can exist with out vlan tagging being set. unlike isl which re-encapsulates the ethernet frame inside an isl frame, adding the 1q header does not render the frame unreadable to non 1q devices. The 1q header is just inserted between the sa and the type/length field in the frame header. many devices that do not make use of 1q vlan tagging still use the 1p portion of the 1q header to mark their cos.
A cisco switch that is setup for mode access will ignore the 1q vlan tagging and put the frame on the vlan that the switchport is configured to. But it will still look at the 1p portion of the 1q header if qos trust cos is configured on the port. Same thing goes for a computer attached to a phone. if the switch is configured to trust the cos marking of the device attached behind the phone, it will look at the 1p portion of the 1q header on the frames coming from the device attached to the phone and respect those values. otherwise it will have the phone remark those values before the frame gets to the switch and get prioritized. <http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a0080094665.shtml#topic1> On Wed, Jul 23, 2008 at 6:57 PM, Mike Brooks <[EMAIL PROTECTED]> wrote: > Its my understanding that the CoS value is set in the 802.1p field > within an 802.1q tag. Therefore, in order to set a CoS value you need > have an 802.1q trunk. So a PC would not be able to set a CoS value, > unless its uplink was an 802.1q trunk port, rather than an access > port. > > So if the PC can't set the CoS value, why would you need to use the > "switchport priority extend cos 0" ? > > Please correct me if I am wrong. > > Regards, > > Mike Brooks > CCIE#16027 (R&S) > > > On 7/23/08, Nick Marus <[EMAIL PROTECTED]> wrote: > > Unless I misunderstand you, COS is applied to a packet and does not > require > > a 1q trunk. The connection between the pc and the phone is not 1q > usually. > > Just the connection between the phone and the switch. Most PC nic's can > be > > setup to mark it's packets with a cos value and effectively take priority > on > > your switched network over you voice and other high priority packets if > the > > switch is trusting and the phone is not remarking to 0. > > > > > > Nick > > > > > > On Wed, Jul 23, 2008 at 10:51 AM, Mike Brooks <[EMAIL PROTECTED]> > wrote: > > > Hi Everyone, > > > > > > I see that the standard practice on a switchport is to configure > > > "switchport priority extend cos 0" in order to allow the ip phone to > > > reset the cos value received from the PC to 0. > > > > > > My question is how would a PC ever set a "CoS" value if the link > > > between the ip phone and the PC is not an 802.1q trunk ? > > > > > > Can someone please help me understand this ? The only thing I can > > > think of is that the PC would somehow have to support an 802.1q trunk > > > to it, a trunk would have to be dynamically established between the > > > phone and PC. And, then the user would have to manipulate the CoS > > > value. Is this possible with a Cisco phone ? > > > > > > If this is the only case this would work then you would think that > > > Cisco would document these pre-requisites. Perhaps I am confused. > > > > > > Please help ;-) > > > > > > Regards, > > > > > > Mike Brooks > > > CCIE#16027 (R&S) > > > > > > > > > > > -- > > Nick Marus > > [EMAIL PROTECTED] > -- Nick Marus [EMAIL PROTECTED]
