From: Edwards, Andrew M (andrew.m.edwards@boeing.com)
Date: Wed Oct 27 2004 - 12:59:49 GMT-3
Actually it confirms what I read, it just isn't very clear on the
DocCD... Go figure.
Basically, what I'm hearing you say, is that with "mls qos trust dscp"
on an access port, the switch trusts incoming CoS values with an
untagged getting 0 (unless I change the default). Then, the switch
writes the DSCP value to the packet according to the cos-to-dscp
mapping. So technically, we aren't dealing with CoS anymore but DSCP.
So if my host is setup to send CoS 3 then the DSCP will be set to 24.
Similarly, I gather, the same holds true for trusting IP precedence.
And just to make sure I really understand, the same holds true for
access or trunk ports to other switches within my QoS domain.
Now, if that isn't correct please let me know.
Thanks,
andy
-----Original Message-----
From: Bob Sinclair [mailto:bsin@cox.net]
Sent: Tuesday, October 26, 2004 8:10 PM
To: Edwards, Andrew M; ccielab@groupstudy.com
Subject: Re: 3550 qos dscp mutation map question
Andy,
As you say, if you trust CoS, then any unmarked (untagged) traffic will
get
the default CoS (0). When that traffic leaves the switch, it will go
out
with the DSCP set according to the cos-dscp map. The default cos-dscp
map
is as follows:
CoS ->DSCP
0 - 0
1 - 8
2 - 16
3 - 24
4 - 32
5 - 40
6 - 48
7 - 56
So, if it comes in with cos 5, it will go out with DSCP marked to 40, by
default. The Auto-QoS feature changes this map so that cos3 gets mapped
to
DSCP 26 (AF31) and cos 5 gets mapped to DSCP 46 (EF). I believe the
dscp-mutation map assumes dscp is trusted. If you wanted to convert
incoming DSCP 40 to outgoing DSCP 46 (EF) you might use this map. Not
sure
if this relates directly to your question, but hope it helps.
Bob Sinclair
CCIE #10427, CISSP, MCSE
www.netmasterclass.net
----- Original Message -----
From: "Edwards, Andrew M" <andrew.m.edwards@boeing.com>
To: <ccielab@groupstudy.com>
Sent: Tuesday, October 26, 2004 8:15 PM
Subject: Re: 3550 qos dscp mutation map question
>I haven't been able to drum up a clear answer in my mind on this one...
>
> If I set a port to trust cos, I know that the default COS (or 0) is
> applied to unmarked traffic. What I don't know is if the switchport
> marks the IP packet with the DSCP value to the cos-to-dscp map?
>
> Or, is the DSCP mapping just another way the switch could filter/act
> on the packet?
>
> Any help with this is appreciated.
>
> andy
>
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