Cisco Nexus 9000 Series Configuration Manual page 154

Guidelines and Limitations for Priority Flow Control
• The dynamic load balancing (DLB) based hashing scheme is enabled by default on all internal links of
a linecard. When DLB is enabled, no-drop traffic might experience out-of-order packet delivery when
congestion on internal links occurs and PFC is applied. If applications on the system are sensitive to
out-of-order delivery, you can adjust for this by disabling DLB at the qos-group level. Disable DLB by
using the set dlb-disable action in the QoS policy-maps and the set qos-group action for no-drop classes.
In the following example assume that qos-group 1 is a no-drop class. DLB is disabled for this no-drop
class by adding the set dlb-disable action and the set qos-group action.
switch(config)# policy-map p1
switch(config-pmap-qos)# class c1
switch(config-pmap-c-qos)# set qos-group 1
switch(config-pmap-c-qos)# set dlb-disable
switch(config-pmap-c-qos)# end
switch# show policy-map p1
• For VLAN-tagged packets, priority is assigned based on the 802.1p field in the VLAN tag and takes
precedence over the assigned internal priority (qos-group). DSCP or IP access-list classification cannot
be performed on VLAN-tagged frames.
• For non VLAN-tagged frames, priority is assigned based on the set qos-group action given by the
ingress QoS policy. Classification is based on a QoS policy-allowed match condition such as precedence,
DSCP, or access-list. You must ensure that the pfc-cos value provided in the network-qos policy for
this class is the same as the qos-group value in this case.
• PFC is not supported for the N9K-X9408PC-CFP2 line card on Cisco Nexus 9500 Series switches.
• Beginning with NX-OS 7.0(3)I1(2), link level flow control and PFC are supported on Cisco Nexus 9300
Series switches and line cards that contain the ALE (Application Leaf Engine).
• PFC on mode is used to support the hosts that support PFC but do not support the Data Center Bridging
Capability Exchange Protocol (DCBXP).
• Only an exact match of the no-drop CoS is considered as a successful negotiation of PFC by the DCBXP.
• Beginning with Cisco NX-OS Release 7.0(3)I3(1), DCBXP is supported on the following Cisco Nexus
switches:
Cisco Nexus 9000 Series NX-OS Quality of Service Configuration Guide, Release 7.x
140
Type qos policy-maps
====================
policy-map type qos p1
class c1
set qos-group 1
set dlb-disable
◦ Nexus 9332PQ switch
◦ Nexus 9372PX switch
◦ Nexus 9372PX-E switch
◦ Nexus C9396PX switch
◦ Nexus 9500 Series switches with the following line cards:
◦ X9432PQ
◦ X9464PX
◦ X9464TX
Configuring Priority Flow Control