To complement this last comment from Thompson... take a look a this: "...All of the IP Phones listed above have the ability to mark 802.1Q/p CoS values for both VoIP and call signaling (default values are 5 and 3, respectively). Furthermore, they also have the ability to mark DSCP values for both VoIP and call signaling (current defaults are EF and AF31, respectively; future software releases will change these values to EF and CS3, respectively)..."
This was taken from the page 63 (2-12) from the QOS SRND 3.3 in the description of the cisco IP Phones, it is an interesting lecture. //r.a. On Thu, Jul 24, 2008 at 6:36 AM, Michael Thompson <[EMAIL PROTECTED]>wrote: > folks, > keep one thing in mind in the process of the PC packets getting to the > switch infrastructure. the IP phone is a switch itself, so as packets come > in port 0 (PC) and go out port 1 (to switch) those layer 2 headers are > 'touched'. So, as it gets forwarded upstream (onto a 802.1q trunk, which is > the only place that COS can exist) those packets can be marked > (rewritten) with the appropriate COS value. > > On Wed, Jul 23, 2008 at 10:01 PM, Nick Marus <[EMAIL PROTECTED]> wrote: > >> 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] >> > >
