Cisco Nexus 5000 Series switches support the Quality of Service features that are
described in this guide.
Quality of Service
quality of service (QoS) features allow you to classify the network traffic,
prioritize the traffic flow, and provide congestion avoidance.
The default QoS
configuration on the device provides
lossless service for Fibre
Channel and Fibre Channel over Ethernet (FCoE) traffic and
best-effort service for Ethernet traffic. QoS can be configured to
provide additional classes of service for Ethernet traffic. Cisco NX-OS QoS
features are configured using Cisco Modular QoS CLI (MQC).
Ethernet is a best-effort medium which means that it lacks any form of flow
In the event of congestion or collisions, Ethernet will drop
packets. The higher level protocols detect the missing data and retransmit the
Fibre Channel requires a reliable
transport system that guarantees the delivery of every packet. To properly
support FCoE, Ethernet has been enhanced with a priority flow control (PFC)
mechanism to prevent congestion.
The FCoE QoS must be
configured either if native FC or FCoE or FC and FCoE are in use. The FCoE QoS
must be added even if Ethernet is not configured on the switch.
following commands will enable the default QoS configuration:
switch(config)# system qos
switch(config-sys-qos)# service-policy type queuing input fcoe-default-in-policy
switch(config-sys-qos)# service-policy type queuing output fcoe-default-out-policy
switch(config-sys-qos)# service-policy type qos input fcoe-default-in-policy
switch(config-sys-qos)# service-policy type network-qos fcoe-default-nq-policy
The Cisco Modular
QoS CLI (MQC) provides a standard set of commands for configuring QoS.
You can use MQC to
define additional traffic classes and to configure QoS policies for the whole
system and for individual interfaces. Configuring a QoS policy with MQC
consists of the following steps:
- Define traffic classes.
- Associate policies and
actions with each traffic class.
- Attach policies to logical or
physical interfaces as well as at the global system level.
MQC provides two
command types to define traffic classes and policies:
class map that represents a class of traffic based on packet-matching criteria.
Class maps are referenced in policy maps.
The class map
classifies incoming packets based on matching criteria, such as the IEEE 802.1p
class of service (CoS) value. Unicast and multicast packets are classified.
Defines a policy map
that represents a set of policies to be applied on a class-by-class basis to
The policy map
defines a set of actions to take on the associated traffic class, such as
limiting the bandwidth or dropping packets.
You define the
policy-map object types when you create them:
objects that you can use for system level related actions.
objects that you can use for classification.
objects that you can use for queuing and scheduling.
qos type is the default for the
policy-map commands, but not for the
service-policy which requires that you specify an
You can attach
policies to interfaces or EtherChannels as well as at the global system level
by using the
You can view all or
individual values for MQC objects by using the
An MQC target is an
entity (such as an Ethernet interface) that represents a flow of packets. A
service policy associates a policy map with an MQC target and specifies whether
to apply the policy on incoming or outgoing packets. This mapping enables the
configuration of QoS policies such as marking, bandwidth allocation, buffer
allocation, and so on.
QoS for Traffic Directed to the CPU
The device automatically applies QoS policies to traffic that is directed to the CPU to ensure that the CPU is not flooded with packets. Control traffic, such as bridge protocol data units (BPDU) frames, is given higher priority to ensure delivery.