From: P729 (p729@cox.net)
Date: Thu Apr 22 2004 - 21:52:15 GMT-3
Please see inline...
Regards,
Mas Kato
https://ecardfile.com/id/mkato
----- Original Message -----
From: "ccie2be" <ccie2be@nyc.rr.com>
To: "Group Study" <ccielab@groupstudy.com>
Sent: Thursday, April 22, 2004 8:56 AM
Subject: QoS on 3550
> Hi guys,
>
> I'm trying to understand something which has me confused.
>
> Recall these facts:
>
> At layer 2, only ISL or 802.1q trunks have fields to carry layer 2 QoS
info.
Layer 2 QoS info = CoS (Class of Service), encoded into the 802.1p bits.
>
> Regular ethernet frames don't and can't carry any QoS info.
Layer 3 QoS info = ToS (Type of Service) and can certainly be carried in a
regular Ethernet frame. Related concepts include DiffServ (Differentiated
Services) and DSCP (DiffServ Code Point), PHB (Per-Hop Behaviors) and
IP Precedence.
>
> Given the above,
>
> Q1) Can cos be set on frames coming into or going out of regular access
port
> on a 3550?
CoS can be set or reset (marked) on frames entering an access port and used
internally for queue management, marking ToS, etc. CoS is passed along if
the frame leaves the switch on a trunk or is stripped if it leaves on an
access
port.
>
> Q2) If so, how does this work?
Look into access port default CoS and trust states.
>
> Q3) Can someone confirm that's there's no problem or Gotcha's on setting
> layer 3 QoS on frames coming into or leaving a regular access port?
Different switches, uh, do it differently--sometimes varying with software
and/or
hardware revision. For example, some can trust CoS or override it, but not
change (re-mark) it. Some can only mark certain values. Different switches
also treat the marked traffic differently, for example, some automatically
map CoS values to a queue and it can't be changes, while others don't do
any mapping by default.
>
> Thanks in advanced, Tim
>
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