From: Shanky (shankyz@gmail.com)
Date: Thu Jun 15 2006 - 04:04:58 ART
Thanks Roberto, for taking out your time and explaining it so well.
Shanky
On 6/14/06, Roberto Fernandez <rofernandez@us.telefonica.com> wrote:
>
> Sami,
>
> Let break it down to the components you will find then all the
> combinations. Here we have two things:
>
> 1- The actual indication to the switch of the presence of a "special"
> VLAN (the voice VLAN)
> 2- The QoS treatment the switch will give to that port (both for the
> data and the access VLANs)
>
> The simplest way for a switch port to work is having a VLAN configured
> and dumb host connected to it. In this case the switch's default
> behavior would be trying to convert the port to a trunk, issuing a few
> DTP (Dynamic Trunking Protocol) frames. But as said before, the host is
> dumb and doesn't care for DTP. The switch quickly gives up and the port
> becomes "dynamic access" in the mean time the switch also tries CDP
> hoping to find a Cisco cousin at the other side.
>
> Let bring now a phone and connect it to that switchport. Down to the
> basics a phone could be no different to the dumb host, but the phones we
> care about are a little bit more; must phones are small switches. And if
> you look at them you will see, they usually have two Ethernet ports: One
> for the uplink and one to connect a device (commonly the PC). From now
> on we will speak fo phones that can talk to the switch, (Cisco phones of
> course)
>
> When you connect one of those Cisco phones to the switch, you will
> clearly have now two devices originating traffic. The phone itself,
> originating voice traffic; and the PC originating data traffic. Having
> two kinds of traffic usually leads to the requirement of differentiating
> them and treating them differently through some QoS technology. When the
> switch recognizes the Cisco phone the will engage in some exchange of
> configurations.
>
> Summarizing, we have now:
>
> 1- A fairly smart Layer3 Switch
> 2- A no so dumb IP phone which in turn connects another device
> 3- The need to differentiate at the switch, traffics originating on the
> phone and the PC.
>
> Well, let start recognizing which traffic belongs to the phone and which
> to the PC. Cisco has two methods
>
> a) 802.1q
> b) 802.1p
>
> a) The 802.1q is really straight forward, when you issue the "switchport
> voice vlan XX" command it is actually a command aimed to the phone. The
> switch is telling to the phone: "tag the voice traffic you are sending
> with XX 802.1q VLAN header" the phone will follow the instruction and by
> default will assign also a CoS of 5 to those frames. The traffic coming
> from the PC will come untagged and the switch will need to know which
> VLAN assign to it, well this is the "switchport access vlan YY"
>
> b) The 802.1p works in principle very similar "switchport voice vlan
> dot1p" is similarly an instruction to the phone, and the default values
> are the same (data on VLAN 0 and Cos of 5 for the 802.1p
>
>
> Now let add the QoS options, remember we have two traffics already
> recognized and the configuration until now is as follow (using 802.1q):
>
> Interface Fastethernet 0/1
> switchport access vlan XX
> switchport voice vlan YY
>
> Well, by default the switch won't care about what is coming and will
> apply assume a CoS of 0 of everything coming into the port be it tagged
> or not, this command "mls qos trust qos" will trust what the phone is
> applying to tagged frames (this is the same for 802.1q or 802.1p kind of
> tags. To be able to use mls qos commands at the interface we need to
> enable it globally also. We have now:
>
> !
> mls qos
> !
> Interface Fastethernet 0/1
> switchport access vlan XX
> switchport voice vlan YY
> mls qos trust qos
> !
>
> and the switch now cares about what the phone is tagging as voice. If we
> do not care about the PC hanging from the phone, we could are done; but
> we do... let say this is an important PC a VP's PC and we want to give
> him something better than the default CoS of 0, well we can change the
> default CoS of the switchport:
>
> mls qos cos 3
>
> now let say the VP is smart enough to make his PC mark traffic as CoS 5,
> but 5 is only voice and maybe the CEO... then you can instruct your
> phone to override his marks
>
> switchport priority extend cos 3
>
> and no matter what he does his data will come as Cos 3
>
> or it could be the CEO, then you let them mark himself as he wants
>
> switchport priority extend trust
>
> So depending the treatment you want to give to the PC you can finish
> your configuration with a combination of changing the default CoS for
> the port and a remote trust of override option for the PC port. Let say
> it is the CEO and we trust his marks, but if he forgets to mark, we will
> give him CoS 3.
>
> !
> mls qos
> !
> Interface Fastethernet 0/1
> switchport access vlan XX
> switchport voice vlan YY
> mls qos trust qos
> mls qos cos 3
> switchport priority extend trust
> !
>
> Well, this is a little bit long, and doesn't cover other options fur
> trusting (DSCP, IP-Precedence, Cisco-Phone) but HTH with the basics,
> going through the configuration guide and then the command reference
> (for more options) will give you the rest.
>
> http://www.cisco.com/univercd/cc/td/doc/product/lan/c3550/12225see/scg/s
> wvoip.htm#wp1034347
>
>
> http://www.cisco.com/univercd/cc/td/doc/product/lan/c3550/12225see/cr/cl
> i1.htm#wp1862788
>
>
> Best Regards,
> Roberto
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> Sami
> Sent: Wednesday, June 14, 2006 4:07 AM
> To: ccielab@groupstudy.com
> Subject: Voice VLAN
>
> Group,
>
> Voice VLAN story is really giving me hard time. We have so many experts
> on
> this list , can't somebody explain what is the right way of configuring
> Voice VLAN. I read three different workbooks CCO doc site and each has
> their own solution.
>
> This one from InternetworkExpert,
>
> interface FastEthernet0/7
> switchport access vlan 7
> switchport voice vlan 10
> switchport priority extend cos 1
> mls qos trust cos
>
> these guys say since 3550 port is in dynamic mode , switch automatically
> form a trunk with Cisco Phone and don't require to configure port as a
> trunk
> or access mode and no spanning tree portfast.....
>
> I know in real life things work in a diffrent way , could some one
> please
> calrify which way Cisco expect us to configure Voice VLAN stuff in lab
> so we
> don't loose point in tussel of Trunk /Access/Advanced CDP/portfast
> etc...stuff.
>
> Many thanks !!
>
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