From: Radioactive Frog (pbhatkoti@gmail.com)
Date: Tue Dec 02 2008 - 08:46:51 ARST
Indeed, good discussion mate...
ok lets first define the trunk :)
trunk is to pass multple vlan and it increases the normal ethernet frame
size [8021q tag we call that].
without trunking you can't inject 802.1p in the ethernet frame.
re;
*switchport voice vlan *{*vlan-id | *dot1p *| *none *| *untagged}
http://www.cisco.com/en/US/docs/switches/lan/catalyst3560/software/release/12
.2_25_sea/configuration/guide/swvoip.html
*here is what dot1p part of above command does;
dot1p*Configure the Cisco IP Phone to use 802.1p priority tagging for voice
traffic and to use the default native VLAN (VLAN 0) to carry all traffic. By
default, the Cisco IP Phone forwards the voice traffic with an 802.1p
priority of 5
I think that command should** only activate once you turn the trunking on.
lab it up and see if packets are being tagged by just issueing that one line
(without switchport encapsulation dot1q).
been there done that!!
By default, the Cisco IP Phone forwards the voice traffic with an 802.1Q
priority of 5
that is if you don't put anything after - switchport voice vlan 100 <dot1p>
When I was doing my masters degree, this used to be a big topics to discuss
and most people still confussed about it.
Good one!!
On Tue, Dec 2, 2008 at 9:26 PM, Carlos G Mendioroz <tron@huapi.ba.ar> wrote:
> 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
>>
>>
>> Blogs and organic groups at http://www.ccie.net
>>
>>
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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
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