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Re: IP Phones and trunks. posted 01/26/2008
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Hello Avner,

I was really in doubt of this because all workbooks I did so far requested
me to configure the "switchport voice vlan xx" to allow the 802.1p .

But there is a CoS training program in the PEC - Partner Education
Connection (at The course name is: "Implementing Cisco Quality
of Service (QoS) 2.2 - EXPRESS" . chapter 4 ( classification and marking )
(slide 4-116 ).

The instructor says pretty clear that -  if the switch attached to the IP
PHONE is configured correctly. it is going to detect that a  phone is
attached and it is going to dynamically create a 802.1q trunk between the
switch and the Phone. Why do we need  a trunk between the switch and the
phone ? because a trunk carries traffic for multiple VLANS, and we have two
vlans in play. we have the voice vlan, sometimes called the auxiliary vlan
and we have the data vlan ... -

So this is where my question came from !

Thanks a lot !

On Jan 26, 2008 5:42 AM, Avner Izhar <aizhar@xxxxxxxxxxxxxx> wrote:

> Hi Daniel,
> As far as I know, the Cisco ip phones do not support DTP or VTP.
> If you connect a phone to a switch that has DTP in dynamic desirable, it
> will not negotiate a trunk.
> The recommended way is to configure an access mode port with a single
> tagged vlan,
>  using the 'switchport voice vlan xx' command.
> That way you prevent floods, but allow separate vlan for voice and for
> data.
> Some old switches will require a trunk (the XL family of switches), on
> them it is a good idea to manually allow only two vlans.
> HTH,
> -------------------------------------------
> thanks,
> Avner Izhar
> CCIE#15999 (Voice), CCVP, CCSI# 31623
> Technical Training Manager
> CCBOOTCAMP - A Cisco Sponsored Organization (SO)
> Email: aizhar@xxxxxxxxxxxxxx
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> -----Original Message-----
> From: nobody@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx] On Behalf Of
> Daniel Valle
> Sent: Thursday, January 24, 2008 16:02
> To: ccielab@xxxxxxxxxxxxxx
> Subject: IP Phones and trunks.
> Hi GS,
> I have a question: when an IP phone connects to a switch, by default,
> the switch will automatically establish a trunk with the IPphone.
> Good. I didn' check that yet, but this makes me conclude tha DTP is
> working in the IP phone. Does it also supports VTP ?
> To be clearer, If I configure vtp prunning in my switch domain and
> later on I plug a IP phone in an interface, is that new trunked
> interface prunned by default or will it be receiving
> floods/broadcast/multicast unnecessarily ( and so to solve it I make
> tune the vlan allowed list )?
> Thanks !
> Daniel
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