Cisco WS-C3550-12G Software Configuration Manual page 458

Understanding QoS
Figure 20-3 Classification Flowchart
Read ingress interface
configuration for classification.
IP and
non-IP
traffic
Check if packet came
with CoS label (tag).
Yes
Use Cos Assign default
from frame. port CoS.
Generate DSCP from
CoS-to-DSCP map.
Done
Are there any (more) QoS ACLs
configured for this interface?
Yes
Read next ACL. Is there
a match with a "permit" action?
Yes
Assign the DSCP as
specified by ACL action.
Catalyst 3550 Multilayer Switch Software Configuration Guide
20-6
Start
Trust DSCP (IP traffic).
Use port Trust IP
default precedence
(non-IP traffic). (IP traffic).
No
Generate the DSCP
based on IP precedence
in packet. Use the
IP-precedence-to-DSCP map.
No
No
Assign the default
DSCP (0).
Done
Trust CoS (IP and non-IP traffic).
Assign DSCP identical
to DSCP in packet.
(Optional) Modify the
DSCP by using the
DSCP-to-DSCP-mutation
map.
Done
Check if packet came
with CoS label (tag).
Yes
Generate the DSCP by using
the CoS-to-DSCP map.
Chapter 20
No
Assign the default port
CoS and generate a
DSCP from the
CoS-to-DSCP map.
Done
Configuring QoS
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