Cisco Systems 3550 - page 454

20-2
Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter20 Co nf iguring QoS
Understanding QoS
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 aLayer 3 packet are described
here and shown in Figure20-1:
•Prioritization values in Layer 2 frames:
Layer 2 Inter-Switch Link (ISL) frame headers have a 1-byte User field tha t 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 valu e
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 nativ e 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.
Figure20-1 QoS Classification Layers in Frames and Packets
Note Layer 3 IPv6 packets are treated as non-IP packets and are bridged by the switch.
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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/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