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Software Configuration Guide—Release 12.2(25)EW
OL-6696-01
Chapter 29 Configuring QoS
Overview of QoS
Prioritization
The QoS implementation for this release is based on the DiffServ architecture, an emerging standard
from the Internet Engineering Task Force (IETF). This architecture specifies that each packet is
classified upon entry into the network. The classification is carried in the IP packet header, using 6 bits
from the deprecated IP type of service (TOS) field to carry the classification (class) information.
Classification can also be carried in the Layer 2 frame. These special bits in the Layer 2 frame or
a Layer 3 packet are described here and shown in Figure 29-1:
• Prioritization values in Layer 2 frames:
Layer 2 Inter-Switch Link (ISL) frame headers have a 1-byte User field that carries an IEEE 802.1p
class of service (CoS) value in the three least-significant bits. On interfaces configured as Layer 2
ISL trunks, all traffic is in ISL frames.
Layer 2 802.1Q frame headers have a 2-byte Tag Control Information field that carries the CoS value
in the three most-significant bits, which are called the User Priority bits. On interfaces configured
as Layer 2 802.1Q trunks, all traffic is in 802.1Q frames except for traffic in the native VLAN.
Other frame types cannot carry Layer 2 CoS values.
Layer 2 CoS values range from 0 for low priority to 7 for high priority.
• Prioritization bits in Layer 3 packets:
Layer 3 IP packets can carry either an IP precedence value or a Differentiated Services Code Point
(DSCP) value. QoS supports the use of either value because DSCP values are backward-compatible
with IP precedence values.
IP precedence values range from 0 to 7.
DSCP values range from 0 to 63.
Figure 29-1 QoS Classification Layers in Frames and Packets
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Encapsulated Packet
Layer 2
header
IP header
3 bits used for CoS
Data
Layer 2 ISL Frame
ISL header
(26 bytes)
Encapsulated frame ...
FCS
(4 bytes)
Layer 2 802.1Q/P Frame
Preamble
Start frame
delimiter
DA
Len
SA Tag PT Data FCS
Layer 3 IPv4 Packet
Version
length
To S
(1 byte)
ID Offset TTL Proto FCS IP-SA IP-DA Data
3 bits used for CoS (user priority)
IP precedence or DSCP
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