From: Carlos G Mendioroz (tron@huapi.ba.ar)
Date: Wed Dec 03 2008 - 08:27:34 ARST
Mark,
802.1Q defines a frame format, which is capable of carrying both a QoS
mark (COS) and a VlanID.
We kind of agreed in calling trunk a link which has a multivlan
capability. I'm basically saying that you can setup a port to honour
the COS marking w/o paying attention to VlanID.
I got sidetracked by my sneefer port filtering vlan tags (Intel pro,
changed OS recently, had to reinstall driver, play with registry et al.)
I'll post my findings shortly.
-Carlos
Mark Stephanus Chandra @ 2/12/2008 21:34 -0200 dixit:
> HI Carlos,
>
> Your statement is still confusing, having 802.1p without a trunk, but it
> carried by dot1q. dot1q is a trunk right ?
>
> Someone in the group once said that even we configure the port switchport
> mode access, but when we add voice vlan configuration, the port change to a
> trunk actually.
>
> Is it true ?
>
> Any lab result ?
>
> Regards
>
> Mark Stephanus Chandra
> IT Consultant
>
> -----Original Message-----
> From: Carlos G Mendioroz [mailto:tron@huapi.ba.ar]
> Sent: 02 Desember 2008 18:01
> To: Radioactive Frog
> Cc: mark.chandra@gmail.com; ccielab@groupstudy.com
> Subject: Re: Voice vlan
>
> Hmm, let's clear it up (what you propose, I mean).
> First, what you configure at the switch is not always going to
> change the way the phone does things, and AFAIK, 802.1p as configured
> in the switch is going to affect the way the switch handles the rx
> packets, so no way to see if it works (wrt trunk/no trunk) by inspecting
> packets in the link.
>
> I don't know what you mean by "not working if you don't activate trunk".
> Now that we agree (I hope) that trunk := multi vlan, what I say is that
> you can have the phone use 802.1p (riding 802.1Q frames) to do QOS
> marking without having a trunk, i.e., having both data and voice traffic
> on same vlan (or not using the vlan id in the 802.1Q frame to actually
> switch the voice packet).
>
> I have not labbed this, but I do believe it works like that.
> If you think it does not, I'll go ahead and lab it up.
>
> -Carlos
>
>
> Radioactive Frog @ 2/12/2008 8:46 -0200 dixit:
>> 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/1
> 2.2_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
>> <mailto: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>
>> <mailto: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>
>> <mailto: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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> _______________________________________________________________________
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>>
>>
>>
>>
>>
>>
>>
>>
>> -- Carlos G Mendioroz <tron@huapi.ba.ar
>> <mailto:tron@huapi.ba.ar> <mailto:tron@huapi.ba.ar
>> <mailto:tron@huapi.ba.ar>>>
>> LW7 EQI Argentina
>>
>>
>>
>> --
>> Carlos G Mendioroz <tron@huapi.ba.ar <mailto:tron@huapi.ba.ar>>
>> LW7 EQI Argentina
>>
>>
>
-- Carlos G Mendioroz <tron@huapi.ba.ar> LW7 EQI ArgentinaBlogs and organic groups at http://www.ccie.net
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