From: Howard C. Berkowitz (hcb@gettcomm.com)
Date: Wed May 05 2004 - 21:03:45 GMT-3
At 6:29 PM -0400 5/5/04, MMoniz wrote:
>Remember though this is Cisco's implementation...Not necessarily other
>vendors.
>
>From a Lab standpoint I would say if the question states:config port x for
>voice with all traffic in the
>same VLAN but voice has a higher priority. Then I would use 802.1p
>
>If it states voice and data should be seperate, then I would use 802.1q
>
If it says voice and data are separate, at least to reflect some
glimmering of reality, I think you have to use both. Otherwise, what
advantage do you get by simply separating them without the trunking
system having any way to prioritize them?
Of course, it can be an extremely silly act to assume there is a
glimmering of reality in possible lab questions.
>
>-----Original Message-----
>From: Kenneth Wygand [mailto:KWygand@customonline.com]
>Sent: Wednesday, May 05, 2004 5:51 PM
>To: MMoniz; rich@myhomemail.net; ccielab@groupstudy.com
>Subject: RE: 802.1q/p
>
>
>So 802.1P is a field in an ISL frame and _not_ in an 802.1q frame? I
>thought it was the other way around... ?????
There is a single field, extended from 802.3, that contains both
802.1p and 802.1q information. Since ISL prepends the VLAN header to
an arbitrary frame, rather than changes the header as in 802.1q,
there's no theoretical reason why you couldn't build an 802.1q/p
frame with just the p bits set, and then carry it in ISL. This would
be vaguely analogous to q-in-q tunneling.
Of course, if you are using ISL, as Mike points out, you have COS
bits available. Could you configure an ingress switch port as ISL and
an egress port as dot1q, and have the COS bits moved from the ISL
header to the Tag Information Control Field?
I have no idea how to even start configuring p-in-ISL, even if it is
supported by IOS as opposed to theoretically possible.
>
>
>-----Original Message-----
>From: MMoniz [mailto:ccie2002@tampabay.rr.com]
>Sent: Wednesday, May 05, 2004 5:45 PM
>To: Kenneth Wygand; rich@myhomemail.net; ccielab@groupstudy.com
>Subject: RE: 802.1q/p
>
>One thing to note is the order of bits for 802.1q and 802.1p for COS
>
>Layer 2 Inter-Switch Link (ISL) frame headers have a 1-byte User field
>that
>carries an IEEE 802.1P class of service (CoS) value in the three
>least-significant bits. On interfaces configured as Layer 2 ISL trunks,
>all
>traffic is in ISL frames.
OK. That's a different QoS than is carried in the Tag Control
Information Field.
>
>Layer 2 802.1Q frame headers have a 2-byte Tag Control Information field
>that carries the CoS value in the three most-significant bits, which are
>called the User Priority bits. On interfaces configured as Layer 2
>802.1Q
>trunks, all traffic is in 802.1Q frames except for traffic in the native
>VLAN.
>
>In older switches, namely the 3524 you have to configure a port as a
>trunk
>port and specify encaps for voice. I have never done it with ISL, only
>dot1.q.
>
>One thing I am not certain of is if the Cat 3550 will consider the
>access
>VLAN ID as the default for 802.1p purposes.
>
>But on this output it definitely shows that voice "IS NOT" on a seperate
>VLAN merely 802.1p tagged
>
>
>
> Name: Fa0/6
>Switchport: Enabled
>Administrative Mode: static access
>Operational Mode: down
>Administrative Trunking Encapsulation: negotiate
>Negotiation of Trunking: Off
>Access Mode VLAN: 6 (VLAN_6)
>Trunking Native Mode VLAN: 1 (default)
>Voice VLAN: dot1p
>
>Maybe someone can clarify?
>
>mike
>
>
>
>-----Original Message-----
>From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
>Kenneth Wygand
>Sent: Wednesday, May 05, 2004 5:18 PM
>To: rich@myhomemail.net; ccielab@groupstudy.com
>Subject: RE: 802.1q/p
>
>
>OK, I think I understand this correctly now. Maybe someone can confirm.
>
>If I set the following configuration on a Cat3550:
>
><snip>
>mls qos
>
>Interface FastEthernet0/1
> switchport access vlan 10
> mls qos trust cos
> switchport voice vlan 20
></snip>
>
>Then my voice packets will be on VLAN 20 and will be tagged with a COS
>value of 5 in the Dot1q frame header (technically considered the Dot1p
>priority bits).
>
>Now if I set the following configuration instead:
>
><snip>
>mls qos
>
>Interface FastEthernet0/1
> switchport access vlan 10
> mls qos trust cos
> switchport voice vlan dot1p
></snip>
>
>Then my voice packets will be on the native VLAN, which is not defined
>in my configuration so should default to VLAN 1. The voice frames will
>once again be tagged with a COS value of 5 in the Dot1q frame header
>(technically considered the Dot1p priority bits).
>
>Now I don't see anywhere in the cisco documentation that I have to
>define a particular encapsulation type on the interface, let alone set
>the native VLAN. Is it possible to use ISL encapsulation on voice
>vlans?
>
>
>-----Original Message-----
>From: Richard Davidson [mailto:rich@myhomemail.net]
>Sent: Wednesday, May 05, 2004 4:07 PM
>To: Kenneth Wygand; ccielab@groupstudy.com
>Subject: Re: 802.1q/p
>
>Yes, Dot1Q is a trunking protocol. When the link is
>trunked the trunk header is placed on the frame. When
>you want to mark some traffic with a higher layer 2
>priority you adjust the 3 bit dot1p field that is in
>the Dot1Q header.
>
>http://www.cisco.com/univercd/cc/td/doc/product/lan/c3550/12119ea1/3550s
>cg/swqos.htm#1032169
>Rich
>
>--- Kenneth Wygand <KWygand@customonline.com> wrote:
>> Is 802.1p just the ISO standard nomenclature for the
>> IP COS bits in an
>> 802.1q frame? In other words, if a question says
>> "make sure to use
>> 802.1p for QoS" or "make sure to set high priority
>> with 802.1p", does
>> this just mean make sure to set the COS bits within
>> an 802.1q frame?
>>
>>
>>
>> Sorry if this seems like a basic question, but I
>> can't find any
>> documentation that specifically solidifies this
>> concept. I just want to
>> make sure that I completely understand what is
>> implied when a
> > requirement is presented to employ 802.1p.
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