From: Chris Lewis (chrlewiscsco@yahoo.com)
Date: Sun Oct 23 2005 - 13:31:29 GMT-3
Javi,
I'd recommend the Catalyst QoS book by Flanagan from Cisco Press, its has about 50 pages devoted to 3550 QoS and tells you all you need to know for the exam regarding QoS on this device.
The central concept here is the internal DSCP. How does it get set, and what does it affect in the output packet.
With no QoS configuration cos and dscp in an ingress packet are untouched by the 3550
If you configure just the global mls qos, the 3550 will mark all cos and dscp to 0 at ingress
If you configure mls qos trust dscp, the dscp of the incoming packet is copeid in to the internal DSCP. All QoS, like policing is performed on the internal DSCP and the resultant internal DSCP value is copied to the cos and DSCP of the egress packet (it may be a different value to the ingress value if it was an exceeding packet for example).
The commands you are referrfing to change the default cos to dscp map table, they do not refer to mapping values within a packet. By default cos 0 maps to dscp 0, cos 1 maps to dscp 8, cos 2 to dscp 16 etc. by changing this default all you are saying is don't map cos 1 to dscp 8, map it to this new value I've configured. However, ubnless you have confiured a trust staet, it is meaningless.
Once you globally configure mls qos on the 3550, you need to define a trust state, either cos or dscp, otherwise the 3550 will not trust anything and all cos and dscp will be set to 0.
Once you are comfortable with this, look at customization by the following commands
Mls qos trust cos pass-through dscp to leave DSCP unchanged.
Mls qos trust dscp pass-through cos leaves CoS unchanged
and
DSCP mutation allows the switch to map ingress DSCP values to alternate configured DSCP values. The map is defined thus
Mls qos map dscp-mutation MAP1 40 41 42 43 to 30
Mls qos map dscp-mutation MAP1 20 21 22 23 to 10
and applied on I/F
int fa0/15
mls qos trust dscp
Mls qos dscp-mutation MAP1
Chris
Javier Tomi <fjtm@tid.es> wrote:
Hi group,
Trying to understand how QoS mapping works on the 3550. I have been
searching through the archives and I don't find a clear answer to this
question. Does "mls qos map cos-dscp" actually write the DSCP value of
ip traffic going through the switch, or that mapping is just used
internally to apply QoS settings?. What about "mls qos map dscp-cos"?
In my scenario I have the following topology:
VLAN2--SW1--VLAN3--R1
SW1 is routing between both VLANs. Traffic from VLAN2 arrives with a
valid CoS and I want to mark DSCP value in such a way that a specific
policy can be applied on R1 based on that DSCP. I was trying the following:
mls qos trust cos (on FE interface of SW1 with VLAN2)
mls qos map cos-dscp
If I would also want traffic on VLAN3 to be mark with an specific CoS
value, Could I use "mls qos map dscp-cos"?
From the documentation it seems that mls qos mapping is just internal
to the switch. If this is true, How could I go to achieve this?
Thank you in advance
Javi Tome
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