Administrator Guide

The pfc on command enables priority-based flow control.
3 Specify the dot1p priority-to-priority group mapping for each priority.
priority-pgid dot1p0_group_num dot1p1_group_num ...dot1p7_group_num
Priority group range is from 0 to 7. All priorities that map to the same queue must be in the same priority group.
Leave a space between each priority group number. For example: priority-pgid 0 0 0 1 2 4 4 4 in which priority group 0 maps to dot1p
priorities 0, 1, and 2; priority group 1 maps to dot1p priority 3; priority group 2 maps to dot1p priority 4; priority group 4 maps to dot1p
priorities 5, 6, and 7.
Dell Networking OS Behavior: As soon as you apply a DCB policy with PFC enabled on an interface, DCBx starts exchanging information
with PFC-enabled peers. The IEEE802.1Qbb, CEE, and CIN versions of PFC Type, Length, Value (TLV) are supported. DCBx also validates
PFC configurations that are received in TLVs from peer devices.
NOTE: You cannot enable PFC and link-level flow control at the same time on an interface.
Configuring Lossless Queues
DCB also supports the manual configuration of lossless queues on an interface when PFC mode is turned off.
Prerequisite: A DCB with PFC configuration is applied to the interface with the following conditions:
• PFC mode is off (no pfc mode on).
• No PFC priority classes are configured (no pfc priority priority-range).
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:
NOTE
: Although, each port on the S4810, S4820T, and S5000 devices support 8 QoS queues, you can configure only 4 QoS
queues (0-3) to manage data traffic. The remaining 4 queues (4-7) are reserved for control traffic.
Dell(conf)#do show qos dot1p-queue-mapping
Dot1p Priority : 0 1 2 3 4 5 6 7
Queue : 0 0 0 1 2 3 3 3
Dell(conf)#
The configuration of no-drop queues provides flexibility for ports on which PFC is not needed but lossless traffic should egress from the
interface.
Lossless traffic egresses out the no-drop queues. Ingress dot1p traffic from PFC-enabled interfaces is automatically mapped to the no-
drop egress queues.
1 Enter INTERFACE Configuration mode.
CONFIGURATION mode
Data Center Bridging (DCB)
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