From: Javier Tomé (fjtm@tid.es)
Date: Sun Oct 23 2005 - 19:27:41 GMT-3
Thank you Chris,
I will try to go through Flanagan's book, but I am scheduled for the 7th
of November, so I am running out of time :-( ... . In the meanwhile I
have been reading the QoS chapter of the Catalyst 3550 configuration
guide. I think I understand the Internal DSCP concept, and how things
work when the 3550 is switching traffic between VLANs.
The flow is as follows:
On the ingress...
Classification (this is where the switch assign internal DSCP) -->
Policying --> Marking
On the egress...
Queuing and Scheduling...
After some test I have found that for IP traffic, packet modification
(i.e. real DSCP marking) also takes place. The DSCP value written in the
packet is the internal DSCP derived from the default or modified mapping
tables (cos-dscp, and so on) unless :
* something different is specified on the Policying/Marking stage
* DSCP mutation map is configured
* pass-through DSCP is configured on ingress interface
* .... (for sure there are more exceptions)
I guess output packets are also marked with a CoS value in case the
output interface is a trunk, although I have not corroborate this.
This also stands when the switch is making L3 through SVI (not sure
about routed interfaces)
Any thoughts?...
Javi
Chris Lewis wrote:
> 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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