Re: Voice vlan

From: Carlos G Mendioroz (tron@huapi.ba.ar)
Date: Tue Dec 02 2008 - 08:26:55 ARST

For the strong disagreement, I guess we have to polish what we mean
for "trunk".

My view is a link carrying many vlans. A link with just one vlan, even
if using 802.1q/p is not really a trunk (again, my view).

This is in line (if not forced by) the way you actually configure
cisco switches where you can enable 802.1p rx by doing
        (config)# switchport voice vlan dot1p

-Carlos

Radioactive Frog @ 2/12/2008 8:04 -0200 dixit:
>
>
> On Tue, Dec 2, 2008 at 8:50 PM, Carlos G Mendioroz <tron@huapi.ba.ar
> <mailto:tron@huapi.ba.ar>> wrote:
>
> Well, not quite.
> Even if *link* is access, phone can mark TOS.
>
>
> Agreed....TOS = layer3
>
>
> Even if *link* is access, phone can use 802.1p to mark COS
>
>
> strongly disagreed as 802.1p can only go with Trunk when packet length
> is scratched a bit using tagging.
> in other word 802.1p is part of 802.1q or 1d frame format: (3bit field
> of 802.1q header is used byu 802.1p).
>
> Check the 802.1q frame header. If you don't tag it won't can't carry 1p bit.
>
>
>
> 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
> <mailto: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 <mailto:tron@huapi.ba.ar>>
> LW7 EQI Argentina
>
> 

-- 
Carlos G Mendioroz  <tron@huapi.ba.ar>  LW7 EQI  Argentina

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

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