From: David Bombal (davidbombal@davidbombal.com)
Date: Tue Jun 06 2006 - 16:34:41 ART
Hi Tony,
A Cisco IP Phone marks 802.1p with a COS of 5 and DSCP of EF by default -
the phone however does not know which the switch is using. COS can only be
used if the port is a trunk (using 8021.1q or ISL encapsulation), so if a
video stream were being sent from a server, it would have to use DSCP if the
server NIC did not support 802.1q/ISL. Thus, trusting CoS is only meaningful
on trunking interfaces, and trusting DSCP (or IP precedence) makes sense for
IP packets only. IP precedence has been replaced with DSCP for marking, but
backward compatibility is available and thus the option to choose IP
Precedence.
A purely layer 2 switch would not be able to read the DSCP value and would
thus be configured to trust COS.
An important point to remember about the Catalyst 3550 is that QoS is
disabled by default and must be enabled globally for configured policies to
become effective. While QoS is disabled, all frames/packets are passed
through the switch unaltered (which is equivalent to a trust CoS and trust
DSCP state on all ports). When QoS is globally enabled however, all DSCP and
CoS values are (by default) set to 0 (which is equivalent to an untrusted
state on all ports).
A 3550 has an internal mapping table between COS and DSCP and vice versa
allows the switch to mark COS & DSCP on egress. Thus for example, if it is
set to trust a COS of 5 on ingress, it will automatically mark the DSCP to
EF (if the table is set accordingly) on egress.
If you entered the auto qos voip trust command, the switch automatically
sets the ingress classification to trust the CoS value received in the
packet on a nonrouted port or to trust the DSCP value received in the packet
on a routed port.
A 3550 switch can thus be set to trust either the COS or the DSCP value that
the phone is marking.
I hope this helps.
David Bombal
CCIE, CCSI, CCIP, CCSP, CCDP
http://www.ConfigureTerminal.com
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of Tony
Paterra
Sent: 05 June 2006 23:27
To: Cisco certification
Subject: [Bulk] mls qos trust command...
When playing with this command on a 3550, I noticed that it only allows you
to trust cos OR dscp OR ip prec OR a device. Why would it not allow for
trusting multiple markings? Am I missing something fundamental here?
If you have an endpoint, say a video stream or RTP conversation. What is
the common practice for marking packets? Would say for instance a 7960
phone mark the COS bits in the 802.1p header as well as the TOS byte in the
L3 header?
Thanks in advance,
-- Tony Paterra apaterra@gmail.com
This archive was generated by hypermail 2.1.4 : Sat Jul 01 2006 - 07:57:32 ART