© 2004 Cisco Systems, Inc. All rights reserved. Printed in USA.
Presentation_ID.scr
109109109
© 2004 Cisco Systems, Inc. All rights reserved.
RST-3508
9805_05_2004_c2
COS/DSCP Mapping
• If a L2 trunk port is configured to "trust dscp",
If the packet is an IP packet, then it will use the DSCP from the IP header
If not, it will use the port default DSCP (configured via qos dscp <val>)
• If a L2 trunk port is configured to "trust cos",
If the packet is tagged it will map the CoS to an internal DSCP (as per CoS-to-DSCP
mapping table) and then use the DSCP-to-TxQ mapping to determine queue and the
DSCP-to-CoS table to determine egress CoS
• If the packet is untagged, it uses the port default CoS and then the
other mapping tables as explained above
Cat4500(config)# qos map cos 7 to dscp 40
r3_4507R_S4#sh qos maps cos dscp
CoS-DSCP Mapping Table
CoS: 0 1 2 3 4 5 6 7
--------------------------------
DSCP: 0 8 16 24 32 40 48 40
Cat4500(config)#qos map dscp 40 to cos 7
Cat4500#sh qos maps dscp cos( truncated)
DSCP-CoS Mapping Table (dscp = d1d2)
d1 : d2 0 1 2 3 4 5 6 7 8 9
-------------------------------------
4 : 07 05 05 05 05 05 05 05 06 06
5 : 06 06 06 06 06 06 07 07 07 07
http://www.cisco.com/en/US/products/hw/switches/ps4324/products_configuration_guide_chapter09186a00801cddc9.html#1223900
Check if COS to DSCP and DSCP to COS Mapping
Is Configured Properly
110110110
© 2004 Cisco Systems, Inc. All rights reserved.
RST-3508
9805_05_2004_c2
Classification/TOS Re-Write Summary
• Determine the “internal DSCP” value—this
depends on the policy-map and port trust
configuration
• If a packet encounters both input and
output classification policy:
Output policy has precedence
If no output policy then input policy has
precedence
If no output/input policy then RX port trust
is used
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