5-4
Quality of Service: Managing Bandwidth More Effectively
QoS Terminology
Classifier-based QoS policies provide greater control for managing network
traffic. Using multiple match criteria, you can finely select and define the
classes of traffic that you want to manage. QoS-specific actions determine
how you can handle the selected traffic.
QoS Terminology
Term Use in This Document
802.1p priority A traffic priority setting carried by a VLAN-tagged packet moving from one device to another through
ports that are tagged members of the VLAN to which the packet belongs. An 802.1p priority can be from
0 to 7. The switch handles an outbound packet on the basis of its 802.1p priority, using the priority to
assign the packet to the appropriate outbound port queue. If a packet leaves the switch on a VLAN
port that is an untagged member of the VLAN, the priority is dropped and the packet arrives at the next,
downstream device without an 802.1p priority assignment.
802.1Q field A four-byte field contained in the header of Ethernet packets entering or leaving the switch through a
port that is a tagged member of a VLAN. This field includes an 802.1p priority setting, a VLAN tag, or ID
number (VID), and other data. A packet entering or leaving the switch through a port that is an untagged
member of the outbound VLAN does not have this field in its header and thus does not carry a VID or
an 802.1p priority. See also “802.1p priority”.
class Defines a set of different types of traffic (using match and ignore commands) to be managed in a
specific way (for example, prioritizing all High Availability traffic).
classifier Method of matching or ignoring certain traffic attributes that allows you to classify packets using more
than one traffic attribute at a time. This method can be used by multiple features, such as QoS and
mirroring.
classifier model Applied to various software features, this configuration model requires you to first classify the traffic
that you want to manage, and then configure a policy containing the actions to be executed on the class.
codepoint See DSCP, below.
CoS Class of Service. Priority level (0-7) used to transmit a packet on an outbound queue. For an individual
packet, class of service is determined by the Layer 2 802.1p priority value in a packet header. The 802.1p
priority is associated with a Layer 3 IP precedence bit-value or DSCP codepoint (see Table 5-11).
Differentiated
Services (Diff-
Serv) mode
See Type-of-Service (ToS), below.
downstream
device
A device linked directly or indirectly to an outbound switch port. The switch sends traffic to
downstream
devices.
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