Cisco IE 3000 Software Configuration Manual page 570

Understanding QoS
The QoS implementation is based on the Differentiated Services (Diff-Serv) 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
Prioritization bits 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 ports configured as Layer 2 ISL
trunks, all traffic is in ISL frames.
Layer 2 IEEE 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 ports configured
as Layer 2 IEEE 802.1Q trunks, all traffic is in IEEE 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 32-1
Cisco IE 3000 Switch Software Configuration Guide
32-2
32-1:
QoS Classification Layers in Frames and Packets
Encapsulated Packet
Layer 2
IP header
header
Layer 2 ISL Frame
ISL header Encapsulated frame 1...
(26 bytes)
3 bits used for CoS
Layer 2 802.1Q and 802.1p Frame
Start frame
Preamble
delimiter
Layer 3 IPv4 Packet
Version ToS
length (1 byte)
Data
(24.5 KB) (4 bytes)
DA SA Tag PT
3 bits used for CoS (user priority)
Len ID Offset TTL Proto FCS IP-SA IP-DA Data
IP precedence or DSCP
Chapter 32
FCS
Data FCS
Configuring QoS
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