Cisco IE 3000 User Manual

32-2
Cisco IE 3000 Switch Software Configuration Guide
OL-13018-01
Chapter 32 Configuring QoS
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 32-1:
• 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 QoS Classification Layers in Frames and Packets
46974
Encapsulated Packet
Layer 2
header
IP header
3 bits used for CoS
Data
Layer 2 ISL Frame
ISL header
(26 bytes)
Encapsulated frame 1...
(24.5 KB)
FCS
(4 bytes)
Layer 2 802.1Q and 802.1p 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

Cisco IE 3000 Specifications

General

Brand	Cisco
Model	IE 3000
Category	Switch
Language	English