The special bits in the Layer 2 frame or a Layer 3 packet are shown in the following figure:
Figure 52: QoS Classification Layers in Frames and Packets
Layer 2 Frame Prioritization Bits
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.
Layer 3 Packet Prioritization Bits
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.
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
543
Information About QoS
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