Re: Voice vlan

From: Carlos G Mendioroz (tron@huapi.ba.ar)
Date: Tue Dec 02 2008 - 07:50:47 ARST

Well, not quite.
Even if *link* is access, phone can mark TOS.
Even if *link* is access, phone can use 802.1p to mark COS

And even is *port* is admin access, it will turn stealthy to trunk
when it discovers a (CDP enabled ?) phone is voice vlan is there:

Switch#sh int f0/8 switchport
Name: Fa0/8
Switchport: Enabled
Administrative Mode: dynamic desirable
Operational Mode: static access
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: native
Negotiation of Trunking: On
Access Mode VLAN: 2 (Lab)
Trunking Native Mode VLAN: 1 (default)
Voice VLAN: 1 (default)
...

(in modern switches at least, it used not to be that way)
-Carlos

Radioactive Frog @ 2/12/2008 5:41 -0200 dixit:
> The idea of voice vlan is phone generates ef or cs3 streams and switch
> prioritize that all the way tilll it see's server/endpoint.
>
> priority field only can be read if the packet is trunked (injected extra
> header in the frame),. If thats link from switch to phone is not trunked
> switch wont' know what kind of frame the phone is sending.
>
> so back to yoru first question, if you applying 'voice vlan under interface'
> then it has to be a trunk port.
>
> if port is access port then phone can't mark its frames with any cos/tos
> bits.
>
> hth...
>
> -frog
> CCIE#21569
>
> On Tue, Dec 2, 2008 at 5:59 PM, <mark.chandra@gmail.com> wrote:
>
>> Dear groups,
>>
>> If we said switchport voice vlan under the interface, it's mean the port
>> become a trunk ?
>>
>> When we have switchport access vlan also, which one is tag with vlan header
>> after exiting the port ? Voice vlan or data vlan ?
>>
>> Thanks group
>> Sent from my BlackBerry. wireless device from XL GPRS/EDGE/3G network
>>
>>
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-- 
Carlos G Mendioroz  <tron@huapi.ba.ar>  LW7 EQI  Argentina

Blogs and organic groups at http://www.ccie.net

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