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Cisco 10000 Series Router Quality of Service Configuration Guide
OL-7433-09
Chapter 7 Marking Traffic
IP Differentiated Services Code Point Marking
You can configure a QoS policy to include an IP DSCP marking for packets entering the network.
Devices within your network can then use the newly marked IP DSCP values to determine how to treat
the packets. For example, class-based weighted random early detection (WRED) uses IP DSCP values
to determine the probability that a packet is dropped. You can also mark voice packets with a particular
DSCP value. You can then configure low-latency queuing (LLQ) to place all packets of that DSCP value
into the priority queue.
DSCP-Based Weighted Random Early Detection
When you configure DSCP-based weighted random early detection (WRED) on an output policy map
and the outgoing packets are MPLS packets, the router drops the MPLS packets based on the three
experimental (EXP) bits in the MPLS label, instead of using the 6-bit DSCP field in the underlying IP
packets. The router shifts the three EXP bits to the left to make it six bits. For example, if the value of
the EXP bits is 5 (binary 101), the router left-shifts the bits to make them binary 101000, thus making it
look like a 6-bit DSCP field. The router drops packets based on the shifted binary value.
set ip dscp Command
To mark a packet by setting the differentiated services code point (DSCP) value in the type of
service
(ToS) byte, use the set ip dscp command in policy-map class configuration mode. To remove a
previously set DSCP value, use the no form of this command. By default, no packets are marked.
set ip dscp {dscp-value | afxy | csx | ef | default}
no set ip dscp {dscp-value | afxy | csx | ef | default}
Syntax Description
ip Specifies that the match is for IPv4 packets only. You must specify this
keyword.
dscp dscp-value Sets the DSCP value. Valid values are from 0 to 63.
Instead of specifying a numeric dscp-value, you can specify one of the
following reserved keywords:
• afxy indicates assured forwarding.
• csx indicates class selector code points that are backward-compatible
with IP precedence. These code points (CS1 through CS7) are identical
to IP precedence values 1 through 7.
• ef indicates expedited forwarding.
• default indicates best effort or DSCP 0.
For more information, see Table 7-4 on page 7-8.
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