Setup Guide

Conguring PFC in a DCB Map
A switch supports the use of a DCB map in which you congure priority-based ow control (PFC) setting. To congure PFC parameters,
you must apply a DCB map on an interface.
PFC Conguration Notes
PFC provides ow control based on the 802.1p priorities in a converged Ethernet trac that is received on an interface and is enabled by
default when you enable DCB. As an enhancement to the existing Ethernet pause functionality, PFC stops trac transmission for specied
priorities (CoS values) without impacting other priority classes. Dierent trac types are assigned to dierent priority classes.
When trac congestion occurs, PFC sends a pause frame to a peer device with the CoS priority values of the trac 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 trac that requires no-drop service, while at the same time retaining packet-drop congestion management for LAN
trac.
On a switch, PFC is enabled by default on Ethernet ports (pfc mode on command). You can congure PFC parameters using a DCB
map or the pfc priority command in Interface conguration mode. For more information, see Conguring 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 congurations that are received in TLVs from peer devices. By
applying a DCB map with PFC enabled, you enable PFC operations on ingress port trac. To achieve complete lossless handling of trac,
congure PFC priorities on all DCB egress ports.
When you apply or remove a DCB input policy from an interface, one or two CRC errors are expected to be noticed on the ingress ports for
each removal or attachment of the policy. This behavior occurs because the port is brought down when PFC is congured. When a DCB
input policy with PFC prole is congured or uncongured on an interface or a range of interfaces not receiving any trac, interfaces with
PFC settings that receive appropriate PFC-enabled trac (unicast, mixed-frame-size trac) display incremental values in the CRC and
discards counters. (These ingress interfaces receiving pfc-enabled trac have an egress interface that has a compatible PFC
conguration).
NOTE
: DCB maps are supported only on physical Ethernet interfaces.
• To remove a DCB map, including the PFC conguration it contains, use the no dcb map command in Interface conguration mode.
• To disable PFC operation on an interface, use the no pfc mode on command in DCB-Map conguration mode.
• Trac may be interrupted when you recongure PFC no-drop priorities in a DCB map or re-apply the DCB map to an interface.
• For PFC to be applied, the congured priority trac 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 ow control on the interface using the flowcontrol
rx on tx on
command. To delete the DCB map, rst disable link-level ow control. PFC is then automatically enabled on the
interface because an interface is PFC-enabled by default.
• To ensure no-drop handling of lossless trac, PFC allows you to congure lossless queues on a port (see Conguring Lossless Queues).
• When you congure 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 ow control is already enabled on an interface. You cannot enable
PFC and link-level ow control at the same time on an interface.
• In a switch stack, congure all stacked ports with the same PFC conguration.
• Dell EMC Networking OS allows you to change the default dot1p priority-queue assignments only if the change satises the following
requirements in DCB maps already applied to the 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.
270
Data Center Bridging (DCB)