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Dell EMC Configuration Guide for the S4048T–ON System 9.14.2.4

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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.
DellEMC(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  
DellEMC(conf)#

DellEMC(conf)#do show qos dot1p-queue-mapping 
 Dot1p Priority : 0  1  2  3  4  5  6  7
          Queue : 2  0  1  3  4  5  6  7  
DellEMC(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
    interface interface-type
  2. Configure the port queues that will still function as no-drop queues for lossless traffic.
    INTERFACE mode
    pfc no-drop queues queue-range
    For the dot1p-queue assignments, refer to the dot1p Priority-Queue Assignment table.
    The maximum number of lossless queues globally supported on the switch is two.
    The range is from 0 to 7. Separate the queue values with a comma; specify a priority range with a dash; for example, pfc no-drop queues 1,7 or pfc no-drop queues 2-7.
    The range is from 0 to 3. Separate the queue values with a comma; specify a priority range with a dash; for example, pfc no-drop queues 1,3 or pfc no-drop queues 2-3.
    The default: No lossless queues are configured.
NOTE Dell EMC Networking OS Behavior: By default, no lossless queues are configured on a port.
A limit of two lossless queues is supported on a port. If the amount of priority traffic that you configure to be paused exceeds the two lossless queues, an error message displays.

It is the user responsibility to have symmetric PFC configurations on the interfaces involved in a particular PFC-enabled traffic-flow to obtain lossless behavior.

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