Mtu; Trust Boundaries - Cisco Nexus 5000 Series Configuration Manual

Configuring QoS
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 2158 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 2158 bytes. This
• 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 DCBX.

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
• By default, all Ethernet interfaces are trusted interfaces.The 802.1p CoS and DSCP are preserved unless
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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 158 bytes. If the Cisco Nexus 5000 Series switch receives an rxbufsize
from a peer that is different than 2158 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 21 58 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
class.
the marking is configured. There is no default CoS to queue and DSCP to queue mapping. You can
define and apply a policy to create these mappings. By default, without a user defined policy, all traffic
is assigned to the default queue.
Cisco Nexus 5000 Series NX-OS Quality of Service Configuration Guide
MTU
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