From: TAM (auha84@dsl.pipex.com)
Date: Mon Jul 17 2006 - 18:31:53 ART
Hi All,
In an effort to get my head around the 3550's QoS features I was
wondering if someone could read and confirm/correct my summary below
regarding the marking of a packet during its time with the 3550, Or
perhaps provide a reference to similar matter.
(assume defaults config unless specified, and by 'Packet' I mean L2/3
frame/packet as appropriate).
<START>
Without mls qos (no concept of trust here):
Packet is sent best-effort.
No marking of packets occur (no change to any Layer2/3 bits within the
packet)
With 'mls qos' enabled and ingress port untrusted:
At ingress the Packet is marked with the default cos value (0,
configurable per port) and default DSCP 0 (not the Internal DSCP value,
but the actual L3 DSCP value of the packet is changed)
At Egress the packet is assigned to wrr queue #1 (default for COS 0 &
1), the packet now has a COS value of 0 and a DSCP value of 0
With 'mls qos' enabled and ingress port trusted:
At egress packet will exit with same incoming value of COS/DSCP/IPP as
it arrived with, i.e.
-if port is trusted for COS, outgoing COS value= incoming COS value
-if port is trusted for IPP, outgoing IPP value= incoming IPP value
-if port is trusted for DSCP, outgoing DSCP value= incoming DSCP value
This is due to the use of the internal DSCP value which allows for the
consistency throughout the packet's lifetime
** think I might be missing something here!
<END>
Thanks in advance,
Thomas.
P.S. First post in Groupstudy! Be lurking silently for months,
thoroughly enjoying the list, makes for excellent reading. Aiming for
Lab in September in Brussels.
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