Prioritization - Cisco 4500M Software Manual

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
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•
Figure 29-1 QoS Classification Layers in Frames and Packets
Encapsulated Packet
Layer 2 ISL Frame
ISL header
(26 bytes)
Layer 2 802.1Q/P Frame
Preamble
Layer 3 IPv4 Packet
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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.
Layer 2
IP header
header
Encapsulated frame ...
3 bits used for CoS
Start frame
DA
delimiter
Version ToS
Len
length (1 byte)
IP precedence or DSCP
Figure 29-1:
Data
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 29 Configuring QoS
FCS
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