Cisco Nexus 5000 Series Switch CLI Software Configuration Guide, NX-OS 4.0(1a)N1 (OL-16597-01, January 2009)

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31-3
Cisco Nexus 5000 Series Switch CLI Software Configuration Guide
OL-16597-01
Chapter 31 Configuring QoS
Information About QoS
• Match CoS value
The match CoS value specifies the IEEE 802.1p CoS value to associate with this system class.
• Bandwidth and priority
Sets the bandwidth and priority configuration values for this system class. The system class values
are used as the default values for all interfaces.
Default System Classes
The Cisco Nexus 5000 Series switch provides the following default system classes:
• FCoE system class
All Fibre Channel and FCoE control and data traffic is automatically classified into the FCoE system
class, which provides no-drop service.
This class is created automatically when the system starts up (the class is named class-fcoe in the
CLI). You cannot delete this class, and you can only modify the IEEE 802.1p CoS value to associate
with this class.
The switch classifies packets into the FCoE system class as follows:
– FCoE traffic is classified based on EtherType.
– Native Fibre Channel traffic is classified based on the physical interface type.
• Drop system class
By default, all unicast and multicast Ethernet traffic is classified into the default drop system class.
This class is created automatically when the system starts up (the class is named class-default in
the CLI). You cannot delete this class and you cannot change the CoS value associated with the
default class.
There are two reserved system classes for internal system use.
Link-Level Flow Control
The IEEE 802.3x link-level flow control capability allows a congested receiver to communicate the far
end to pause its data transmission for a short period of time. The link-level flow control feature applies
to all the traffic on the link.
The transmit and receive directions are separately configurable. By default, link-level flow control is
disabled for both directions.
On the Cisco Nexus 5000 Series switch, Ethernet interfaces do not auto-detect the link-level flow control
capability. You must configure the capability explicitly on the Ethernet interfaces.
On each Ethernet interface, the switch can enable either priority flow control or link-level flow control
(but not both).
Priority Flow Control
The priority flow control (PFC) capability allows you to apply pause functionality to specific classes of
traffic on a link (instead of all the traffic on the link). PFC applies pause functionality based on the IEEE
802.1p CoS value. When the switch enables PFC, it communicates to the adapter which CoS values to
apply the pause.