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Dell PowerEdge FN I/O Module Configuration Guide 9.10(0.0)

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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 and priority classes are disabled in a DCB map, apply the map on the interface.

Prerequisite: A DCB input policy 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: 

Dell(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.
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 type slot/port
  2. Configure the port queues that still functions 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 four. 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 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. Reconfigure the input policy using a smaller number of PFC priorities.

If you configure lossless queues on an interface that already has a DCB input policy with PFC enabled ( pfc mode on), an error message displays.

Traffic may be interrupted due to an interface flap (going down and coming up) when you reconfigure lossless queues on no-drop priorities in an input policy and re-apply the policy to an interface.

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