Cisco ASR 901 Command Reference Manual page 141

Chapter 1 CLI Command Reference
Usage Guidelines Once the DSCP bit is set, other quality of service (QoS) features can then operate on the bit settings.
DSCP and Precedence Values Are Mutually Exclusive
The set dscp command cannot be used with the set precedence command to mark the same packet. The
two values, DSCP and precedence, are mutually exclusive. A packet can have one value or the other, but
not both.
Precedence Value and Queueing
The network gives priority (or some type of expedited handling) to marked traffic. Typically, you set the
precedence value at the edge of the network (or administrative domain); data then is queued according
to the precedence. Weighted fair queueing (WFQ) can speed up handling for high-precedence traffic at
congestion points. Weighted Random Early Detection (WRED) ensures that high-precedence traffic has
lower loss rates than other traffic during times of congestion.
Use of the "from-field" Packet-marking Category
If you are using this command as part of the Enhanced Packet Marking feature, you can use this
command to specify the "from-field" packet-marking category to be used for mapping and setting the
DSCP value. The "from-field" packet-marking categories are as follows:
•
•
If you specify a "from-field" category but do not specify the table keyword and the applicable
table-map-name argument, the default action will be to copy the value associated with the "from-field"
category as the DSCP value. For instance, if you configure the set dscp cos command, the CoS value
will be copied and used as the DSCP value.
The CoS field is a three-bit field, and the DSCP field is a six-bit field. If you configure the set dscp cos
Note
command, only the three bits of the CoS field will be used.
If you configure the set dscp qos-group command, the QoS group value will be copied and used as the
DSCP value.
The valid value range for the DSCP is a number from 0 to 63. The valid value range for the QoS group
is a number from 0 to 99. Therefore, when configuring the set dscp qos-group command, note the
following points:
•
•
Set DSCP Values in IPv6 Environments
When this command is used in IPv6 environments, the default match occurs on both IP and IPv6 packets.
However, the actual packets set by this function are only those which meet the match criteria of the
class-map containing this function.
Set DSCP Values for IPv6 Packets Only
To set DSCP values for IPv6 values only, the match protocol ipv6 command must also be used. Without
that command, the precedence match defaults to match both IPv4 and IPv6 packets.
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Class of service (CoS)
QoS group
If a QoS group value falls within both value ranges (for example, 44), the packet-marking value will
be copied and the packets will be marked.
If QoS group value exceeds the DSCP range (for example, 77), the packet-marking value will not
be copied and the packet will not be marked. No action is taken.
Cisco ASR 901 Aggregation Services Router Command Reference Guide
set dscp
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