Reference Guide

When traffic congestion occurs, PFC sends a pause frame to a peer device with the CoS priority values of the traffic that needs
to be stopped. DCBx provides the link-level exchange of PFC parameters between peer devices. PFC allows network
administrators to create zero-loss links for SAN traffic that requires no-drop service, while at the same time retaining packet-
drop congestion management for LAN traffic.
On a Z9500 switch, PFC is enabled by default on Ethernet ports (pfc mode on command). You can configure PFC
parameters using a DCB map or the pfc priority command in Interface configuration mode. For more information, see
Configuring Priority-Based Flow Control.
As soon as you apply a DCB map with PFC enabled on an interface, DCBx starts exchanging information with a peer. The
IEEE802.1Qbb, CEE and CIN versions of PFC 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 operations on ingress port traffic. To
achieve complete lossless handling of traffic, configure PFC priorities on all DCB egress ports.
NOTE: DCB maps are supported only on physical Ethernet interfaces.
● To remove a DCB map, including the PFC configuration it contains, use the no dcb map command in Interface
configuration mode.
● To disable PFC operation on an interface, use the no pfc mode on command in DCB-Map configuration mode.
● Traffic may be interrupted when you reconfigure PFC no-drop priorities in a DCB map or re-apply the DCB map to an
interface.
● For PFC to be applied, the configured priority traffic must be supported by a PFC peer (as detected by DCBx).
● If you apply a DCB map with PFC disabled (pfc off), you can enable link-level flow control on the interface using the
flowcontrol rx on tx on command. To delete the DCB map, first disable link-level flow control. PFC is then
automatically enabled on the interface because an interface is PFC-enabled by default.
● To ensure no-drop handling of lossless traffic, PFC allows you to configure lossless queues on a port (see Configuring
Lossless Queues).
● When you configure a DCB map, an error message is displayed if the PFC dot1p priorities result in more than two lossless
queues.
● When you apply a DCB map, an error message is displayed if link-level flow control is already enabled on an interface. You
cannot 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.
● Dell Networking OS allows you to change the default dot1p priority-queue assignments only if the change satisfies the
following requirements in DCB maps already applied to S6000 interfaces:
● All 802.1p priorities mapped to the same queue must be in the same priority group.
● A maximum of two PFC-enabled, lossless queues are supported on an interface.
Otherwise, the reconfiguration of a default dot1p-queue assignment is rejected.
● To ensure complete no-drop service, apply the same PFC parameters on all PFC-enabled peers.
PFC Prerequisites and Restrictions
On an S6000 switch, PFC is globally enabled by default, but not applied on specific 802.1p priorities. To enable PFC on 802.1p
priorities, create a DCB map.
The following prerequisites and restrictions apply when you configure PFC in a DCB map:
● You can enable PFC on a maximum of two priority queues on an interface. Enabling PFC for dot1p priorities configures the
corresponding port queue as lossless.
● You cannot enable PFC and link-level flow control at the same time on an interface.
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Data Center Bridging (DCB)