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

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Cisco Nexus 5000 Series Switch CLI Software Configuration Guide
OL-16597-01
Chapter 31 Configuring QoS
Information About QoS
Ethernet interfaces use PFC to provide lossless service to no-drop system classes. PFC implements Pause
frames on a per-class basis and uses the IEEE 802.1p CoS value to identify the classes that require
lossless service.
In the switch, each system class has an associated IEEE 802.1p CoS value (assigned by default or
configured on the system class). If PFC is enabled, the switch sends the no-drop CoS values to the
adapter, which then applies PFC to these CoS values.
The default CoS value for the FCoE system class is 3 and this value is configurable. The default CoS
value for the default drop system class is 0 and this value is configurable.
If PFC is not enabled on an interface, you can enable IEEE 802.3X link-level pause. By default,
link-level pause is disabled.
MTU
The Cisco Nexus 5000 Series switch is a Layer 2 switch, and it does not support packet fragmentation.
MTU configuration mismatch between ingress and egress interfaces may result in packets being
truncated.
When configuring MTU, follow these guidelines:
• MTU is specified per system class. You cannot configure MTU on the interfaces.
• Fibre Channel and FCoE payload MTU is 2112 bytes across the switch. As a result, the rxbufsize
for Fibre Channel interfaces is fixed at 2112 bytes. If the Cisco Nexus 5000 Series switch receives
an rxbufsize from a peer different than 2112 bytes, it will fail ELP negotiation and not bring the link
up.
• The system jumbomtu command defines the upper bound of any MTU in the system. System jumbo
MTU has a default value of 9216 bytes. The minimum MTU is 2240 bytes and the maximum MTU
is 9216 bytes.
• The system class MTU sets the MTU for all packets in the class. The system class MTU cannot be
configured larger than the global jumbo MTU.
• The FCoE system class (for Fibre Channel and FCoE traffic) has a default MTU of 2240 bytes. This
value cannot be modified.
• The default drop system class has a default MTU of 1500 bytes. You can configure this value.
• The switch sends the MTU configuration to network adapters that support DCBXP.
Trust Boundaries
The trust boundary is enforced by the incoming interface as follows:
• All Fibre Channel and virtual Fibre Channel interfaces are automatically classified into the FCoE
system class.
• By default, all Ethernet interfaces are trusted interfaces. A packet tagged with a 802.1p CoS value
is classified into a system class using the value in the packet.
• Any packet not tagged with an 802.1p CoS value is classified as if it were marked with a CoS value
of 0. By default CoS 0 maps to the default drop system class.