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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 2 802.1Q 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 2 802.1Q 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 32-1 QoS Classification Layers in Frames and Packets
Encapsulated Packet
Layer 2
header
Layer 2 ISL Frame
ISL header
(26 bytes)
Layer 2 802.1Q/P Frame
Preamble
Layer 3 IPv4 Packet
Version
length
Catalyst 3750 Switch Software Configuration Guide
32-2
32-1:
IP header
Data
Encapsulated frame 1...
(24.5 KB)
3 bits used for CoS
Start frame
DA
delimiter
ToS
Len
(1 byte)
IP precedence or DSCP
FCS
(4 bytes)
SA
Tag
PT
Data
3 bits used for CoS (user priority)
ID
Offset TTL Proto FCS IP-SA IP-DA Data
Chapter 32
FCS
Configuring QoS
78-16180-02
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