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Quality of Service: Managing Bandwidth More Effectively
QoS Operation
Term Use in This Document
Type-of-Service
(ToS)
Traffic Class
byte
upstream
device
One of the bytes in an IPv4 packet header. IPv4 packets may be classified according to two type-of-
service modes:
• IP-precedence mode, using the upper three bits of the ToS byte
• Differentiated Services (Diff-Serv) mode, using the upper six bits of the ToS byte.
For more information about the IPv4 ToS field, see Table 6-11 and the “IP-precedence bits” and “DSCP”
definitions in this table.
In IPv6 packets, the byte that corresponds to the IPv4 Type-of-Service field, consisting of a six-bit (high-
order) Differentiated Services (Diff-Serv) field and a two-bit (low-order) reserved field. The three-bit
(high-order) precedence field in the Traffic Class byte corresponds to the IP precedence bit set in IPv4
packets. See also “IP-precedence bits” and “DSCP” elsewhere in this table.
Note: As in IPv4, IPv6 packets may be classified according to two type-of-service modes:
• IP-precedence mode, using the upper three bits of the Traffic Class byte
• Differentiated Services (Diff-Serv) mode, using the upper six bits of the Traffic Class byte
For more information about the IPv6 Traffic Class field, see Table 6-11 on page 6-89 and the “IP-
precedence bits” and “DSCP” definitions in this table.
A device linked directly or indirectly to an inbound switch port. The switch receives traffic from
upstream devices.
QoS Operation
On the switches covered in this guide, QoS operation may be configured
through a combination of the following methods:
■ Globally-configured, switch-wide QoS settings
■ Classifier-based per-port and per-VLAN QoS policies.
Classifier-based QoS policies are designed to work with existing globally-
configured, switch-wide QoS settings by allowing you to zoom in on a subset
of port or VLAN traffic to further manage it. You can use multiple match
criteria to more finely select and define the classes of traffic that you want to
manage. QoS policy actions determine how you can handle the selected traffic.
Note While providing greater control for implementing QoS policies, classifier-
based QoS policies may override globally-configured QoS settings. For more
information, see “Override of Global QoS Settings” on page 6-81.
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