Cisco Catalyst 2975 Software Configuration Manual page 726

Understanding QoS
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.
Note
Figure 33-1
Encapsulated Packet
Layer 2
header
Layer 2 ISL Frame
ISL header
(26 bytes)
Layer 2 802.1Q and 802.1p Frame
Preamble
Layer 3 IPv4 Packet
Version
length
Catalyst 2975 Switch Software Configuration Guide
33-2
33-1:
IPv6 QoS is not supported in this release.
QoS Classification Layers in Frames and Packets
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 33
FCS
Configuring QoS
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