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

PDF

Configuring Priority-Based Flow Control

PFC provides a flow control mechanism based on the 802.1p priorities in converged Ethernet traffic received on an interface and is enabled by default when you enable DCB.

As an enhancement to the existing Ethernet pause mechanism, PFC stops traffic transmission for specified priorities (Class of Service (CoS) values) without impacting other priority classes. Different traffic types are assigned to different priority classes.

When traffic congestion occurs, PFC sends a pause frame to a peer device with the CoS priority values of the traffic that is to be stopped. Data Center Bridging Exchange protocol (DCBx) provides the link-level exchange of PFC parameters between peer devices. PFC allows network administrators to create zero-loss links for Storage Area Network (SAN) traffic that requires no-drop service, while retaining packet-drop congestion management for Local Area Network (LAN) traffic.

To ensure complete no-drop service, apply the same dcb-map on all PFC and ETS enabled interfaces.

  1. Create a DCB map to apply priority based flow control or enhanced transmission selection for specified priority groups and priorities. CONFIGURATION mode dcb-map map-name The maximum is 32 alphanumeric characters.
  2. Configure the priority group with PGID, bandwidth percentage or strict priority for ETS and PFC mode. DCB-MAP mode priority-group pg_num [bandwidth percentage | strict-priority] pfc [on | off] pg_num range is from 0 to 7. bandwidth percentage range is from 1 to 100. Either strict-priority or bandwidth percentage can be set for ETS on the priority group. PFC can either be enabled or disabled for the priority group.
  3. Configure the priorities to priority group. DCB-MAP mode priority-pgid <pgid> <pgid> <pgid> <pgid> <pgid> <pgid> <pgid> <pgid> pgid range is from 0 to 7. Configure priority to priority group mapping from priority 0 to priority 7 in order.
  4. Exit the DCB MAP configuration mode. DCB-MAP mode exit
  5. Enter interface configuration mode. CONFIGURATION mode interface type slot/port
  6. Apply the dcb-map with PFC and ETS configurations to both ingress and egress interfaces. INTERFACE mode dcb-map map-name
  7. Repeat steps 1 to 6 on all PFC and ETS enabled interfaces to ensure lossless traffic service.
    • NOTE: All these configurations are available only in PMUX mode and you cannot perform these configurations in Standalone mode.
Dell Networking OS Behavior: As soon as you apply a DCB MAPwith 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.

By applying a DCB MAP with PFC enabled, you enable PFC operation on ingress port traffic. To achieve complete lossless handling of traffic, also enable PFC on all DCB egress ports or configure the dot1p priority-queue assignment of PFC priorities to lossless queues.

To remove a DCB MAP, including the PFC and ETS configurations it contains, use the no dcb-map map-name command in INTERFACE Configuration mode.

You can enable any number of 802.1p priorities for PFC. Queues to which PFC priority traffic is mapped are lossless by default. Traffic may be interrupted due to an interface flap (going down and coming up) when you reconfigure the lossless queues for no-drop priorities in a PFC dcb-map and reapply the policy to an interface.

To apply PFC, a PFC peer must support the configured priority traffic (as detected by DCBx).

To honor a PFC pause frame multiplied by the number of PFC-enabled ingress ports, the minimum link delay must be greater than the round-trip transmission time the peer requres.

If you apply dcb-map with PFC disabled ( no pfc mode on):
  • You can enable link-level flow control on the interface. To delete the dcb-map, 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.
  • NOTE: You cannot enable PFC and link-level flow control at the same time on an interface.
When you apply a dcb-map 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 MAP for PFC applied to an interface may become invalid if you reconfigure dot1p-queue mapping. 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 resynchronized with the peer devices.

Traffic may be interrupted when you reconfigure PFC no-drop priorities in a dcb-map or reapply the dcb-map to an interface.

  • NOTE: All these configurations are available only in PMUX mode and you cannot perform these configurations in Standalone mode.

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