Dell EMC Configuration Guide for the S4048T–ON System 9.14.2.4

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Configuring PFC without a DCB Map

In a network topology that uses the default ETS bandwidth allocation (assigns equal bandwidth to each priority), you can also enable PFC for specific dot1p-priorities on individual interfaces without using a DCB map. This type of DCB configuration is useful on interfaces that require PFC for lossless traffic, but do not transmit converged Ethernet traffic.

Table 1. Configuring PFC without a DCB MapConfiguring PFC without a DCB Map
Step	Task	Command	Command Mode
1	Enter interface configuration mode on an Ethernet port.	interface interface-type}	CONFIGURATION
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 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.

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.

Table 2. Configuring Lossless Queues on a Port InterfaceConfiguring Lossless Queues on a Port Interface
Step	Task	Command	Command Mode
1	Enter INTERFACE Configuration mode.	interface interface-type	CONFIGURATION
2	Open a DCB map and enter DCB map configuration mode.	dcb-map name	INTERFACE
3	Disable PFC.	no pfc mode on	DCB MAP
4	Return to interface configuration mode.	exit	DCB MAP
5	Apply the DCB map, created to disable the PFC operation, on the interface	dcb-map {name \| default}	INTERFACE
6	Configure the port queues that still function as no-drop queues for lossless traffic. For the dot1p-queue assignments. The maximum number of lossless queues globally supported on a port is 2. You cannot configure PFC no-drop queues on an interface on which a DCB map with PFC enabled has been applied, or which is already configured for PFC using the pfc priority command. Range: 0-3. Separate 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 Default: No lossless queues are configured.	pfc no-drop queuesqueue-range	INTERFACE