Dell PowerSwitch S4820T 9.50.1 Configuration Guide for the S4820T System - Page 273
Configuring Lossless Queues, Ethernet Pause Frames, Quality of Service QoS
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If you apply an input policy with PFC disabled (no pfc mode on): • You can enable link-level flow control on the interface (refer to Ethernet Pause Frames). To delete the input policy, first disable link-level flow control. PFC is then automatically enabled on the interface because an interface is by default PFC-enabled. • PFC still allows you to configure lossless queues on a port to ensure no-drop handling of lossless traffic (refer to Configuring Lossless Queues). NOTE: You cannot enable PFC and link-level flow control at the same time on an interface. When you apply an input policy to an interface, an error message displays if: • The PFC dot1p priorities result in more than two lossless port queues globally on the switch. • Link-level flow control is already enabled. You cannot be enable PFC and link-level flow control at the same time on an interface. • In a switch stack, configure all stacked ports with the same PFC configuration. A DCB input policy for PFC applied to an interface may become invalid if you reconfigure dot1p-queue mapping (refer to the Create Input Policy Maps section in the Quality of Service (QoS) chapter). This situation occurs when the new dot1p-queue assignment exceeds the maximum number (2) of lossless queues supported globally on the switch. In this case, all PFC configurations received from PFC-enabled peers are removed and re-synchronized with the peer devices. Traffic may be interrupted when you reconfigure PFC no-drop priorities in an input policy or reapply the policy to an interface. 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 input policy applied to 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). 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 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. Data Center Bridging (DCB) 273
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