From: Ryan (ryan95842@gmail.com)
Date: Wed Jan 24 2007 - 18:53:36 ART
So unless there are trunk links in the mix, the COS value is pretty much not
going to do anything?
Am I correct though in the DSCP value being copied to the COS where a trunk
link is concerned?
(i.e. Phone is a trunk link to the switch)
Ryan
On 1/24/07, Brian McGahan <bmcgahan@internetworkexpert.com> wrote:
>
> COS only exists in the trunking header of ISL and 802.1q frames,
> it's not part of the regular Ethernet header. DSCP is part of the
> normal IP header. If your voice traffic is not transiting a trunk link
> then looking at the COS won't accomplish anything because it's not
> there. If you look at the DSCP value you can usually assume that VoIP
> will be DSCP EF.
>
>
> HTH,
>
> Brian McGahan, CCIE #8593 (R&S/SP)
> bmcgahan@internetworkexpert.com
>
> Internetwork Expert, Inc.
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> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> Ryan
> Sent: Wednesday, January 24, 2007 2:50 PM
> To: ccielab@groupstudy.com
> Subject: Voice QoS, What's the difference between trusting COS vs DSCP?
>
> Greetings,
>
> I'm trying to understand the difference between a switch trusting a COS
> value vs trusting a DSCP value.
>
> With my current understanding, a phone would generate a packet (L3) and
> mark
> this packet with an appropriate DSCP value. This packet would then be
> encapsulated in a L2 frame as it leaves the phone. The DSCP value would
> be
> copied to the the L2 frame COS value to closest to it. (i.e. 46 DSCP
> would
> become 5 COS by default). This L2 frame would enter the wire and go to
> the
> connected switch.
>
> At this point, the switch has the option to either trust the COS value
> (on
> the L2 frame) or the encapsulated DSCP value in the L3 packet.
>
> I want to understand the ramifications of choosing one over the other.
>
> Based on my current understanding, I would want to trust DSCP as the
> switch
> and all devices along the path would keep the L3 DSCP value intact, and
> strip and reapply the COS value whenever L3 to L2 back to L3
> encapsulation
> occurs. Where by trusting the L2 COS value, as soon as the packet
> reaches a
> L3 interface, the frame is de-encapsulated and the COS is tossed. Since
> we
> are trusting COS, and no COS exists, essentially all QoS is lost at this
> point.
>
> For example (best viewed with fix width font):
>
> DSCP = 46
>
> L3 IP address SVI 5 SVI 5 IP address
> L2 PHONE <-----> SW1 <------> SW2 <-----> SW3 <-----> SBC
> VLAN 5 5 5 5
>
> In this above case, trusting DSCP, the packet would maintain it's QoS
> value
> through the entire network as it passes from L2 to L3 and back. But if
> trust
> COS, SW1 would strip the L2 frame removing the COS value, and
> essentially
> losing any further QoS as we no longer have a value to trust.
>
> Is this correct? Or have I missed something in the behavior?
>
> What's the difference between trusting COS vs DSCP?
>
>
>
>
> Regards,
>
>
> Ryan
>
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