Install Guide

Table 17. Configuring PFC without a DCB Map (continued)
Step Task Command Command Mode
2 Enable PFC on specified priorities. Range: 0-7. Default:
None.
Maximum number of lossless queues supported on an
Ethernet port: 2.
Separate priority values with a comma. Specify a
priority range with a dash, for example: pfc priority
3,5-7
1. You cannot configure PFC using the pfc
priority command on an interface on which a
DCB map has been applied or which is already
configured for lossless queues (pfc no-drop
queues command).
pfc priority priority-
range
INTERFACE
Configuring Lossless Queues
DCB also supports the manual configuration of lossless queues on an interface when PFC mode is disabled in a DCB map, apply
the map on the interface. The configuration of no-drop queues provides flexibility for ports on which PFC is not needed, but
lossless traffic should egress from the interface.
Configuring no-drop queues is applicable only on the interfaces which do not need PFC.
Example:
Port A —> Port B
Port C —> Port B
PFC no-drop queues are configured for queues 1, 2 on Port B. PFC capability is enabled on priorities 3, 4 on PORT A and C.
Port B acting as Egress
During the congestion, [traffic pump on priorities 3 and 4 from PORT A and PORT C is at full line rate], PORT A and C send out
the PFCs to rate the traffic limit. Egress drops are not observed on Port B since traffic flow on priorities is mapped to loss less
queues.
Port B acting as Ingress
If the traffic congestion is on PORT B , Egress DROP is on PORT A or C, as the PFC is not enabled on PORT B.
Refer the following configuration for queue to dot1p mapping:
DellEMC(conf)#do show qos dot1p-queue-mapping
Dot1p Priority : 0 1 2 3 4 5 6 7 -> On ingress interfaces[Port A and C] we used
the PFC on priority level.
Queue : 0 0 0 1 2 3 3 3 -> On Egress interface[Port B] we used no-drop
queues.
Lossless traffic egresses out the no-drop queues. Ingress 802.1p traffic from PFC-enabled peers is automatically mapped to the
no-drop egress queues.
When configuring lossless queues on a port interface, consider the following points:
● By default, no lossless queues are configured on a port.
● A limit of two lossless queues is supported on a port. If the number of lossless queues configured exceeds the maximum
supported limit per port (two), an error message is displayed. Reconfigure the value to a smaller number of queues.
● If you configure lossless queues on an interface that already has a DCB map with PFC enabled (pfc on), an error message is
displayed.
220
Data Center Bridging (DCB)