Trust Boundaries - Cisco AP775A - Nexus Converged Network Switch 5010 Configuration Manual

MTU
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 that is assigned by default or
configured on the system class. If you enable PFC, 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. This value is configurable.
By default, the switch negotiates to enable the PFC capability. If the negotiation succeeds, PFC is enabled
and link-level flow control remains disabled regardless of its configuration settings. If the PFC negotiation
fails, you can either force PFC to be enabled on the interface or you can enable IEEE 802.x link-level flow
control.
If you do not enable PFC 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. A
maximum transmission unit (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. The system class allows a different MTU for each class of traffic
• Fibre Channel and FCoE payload MTU is 2112 bytes across the switch. As a result, the rxbufsize for
• Enter the system jumbomtu command to define the upper bound of any MTU in the system. The system
• The system class MTU sets the MTU for all packets in the class. The system class MTU cannot be
• The FCoE system class (for Fibre Channel and FCoE traffic) has a default MTU of 2240 bytes. This
• The default drop system class has a default MTU of 1538 bytes. You can configure this value.
• The switch sends the MTU configuration to network adapters that support DCBX.

Trust Boundaries

The trust boundary is enforced by the incoming interface as follows:
Cisco Nexus 5000 Series Switch CLI Software Configuration Guide
426
but they must be consistent on all ports across the entire switch. You cannot configure MTU on the
interfaces.
Fibre Channel interfaces is fixed at 2112 bytes. If the Cisco Nexus 5000 Series switch receives an
rxbufsize from a peer that is different than 2112 bytes, it will fail the exchange of link parameters (ELP)
negotiation and not bring the link up.
jumbo MTU has a default value of 9216 bytes. The minimum MTU is 2240 bytes and the maximum
MTU is 9216 bytes.
configured larger than the global jumbo MTU.
value cannot be modified.
MTU is not supported in Converged Enhanced Ethernet (CEE) mode for DCBX.
Note
Information About QoS
OL-16597-01