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Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 37 Configuring Quality of Service About QoS
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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.
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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.
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Other frame types cannot carry Layer 2 CoS values.
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Layer 2 CoS values range from 0 for low priority to 7 for high priority.
• Prioritization bits in Layer 3 packets:
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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.
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IP precedence values range from 0 to 7.
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DSCP values range from 0 to 63.
Figure 37-1 QoS Classification Layers in Frames and Packets
All switches and routers across the Internet rely on the class information to provide the same forwarding
treatment to packets with the same class information and different treatment to packets with different
class information. The class information in the packet can be assigned by end hosts or by switches or
routers along the way, based on a configured policy, detailed examination of the packet, or both. Detailed
examination of the packet is expected to happen closer to the edge of the network so that the core
switches and routers are not overloaded.
68140
Encapsulated Packet
Layer 2
header IP header
3 bits used for CoS
Data
Layer 2 ISL Frame
ISL header
(26 bytes) Encapsulated frame ... FCS
(4 bytes)
Layer 2 802.1Q/P Frame
Preamble Start frame
delimiter DA
Len
SA Tag PT Data FCS
Layer 3 IPv4 Packet
Version
length ToS
(1 byte) ID Offset TTL Proto FCS IP-SA IP-DA Data
3 bits used for CoS (user priority)
IP precedence or DSCP