From: Brian McGahan (bmcgahan@internetworkexpert.com)
Date: Wed Jan 24 2007 - 18:28:02 ART
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.
http://www.InternetworkExpert.com
Toll Free: 877-224-8987 x 705
Outside US: 775-826-4344 x 705
24/7 Support: http://forum.internetworkexpert.com
Live Chat: http://www.internetworkexpert.com/chat/
-----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
This archive was generated by hypermail 2.1.4 : Thu Feb 08 2007 - 23:46:57 ART