Concept Guide

As a result, PFC and lossless port queues are disabled on 802.1p priorities, and all priorities are mapped to the same priority queue and
equally share the port bandwidth.
• To change the ETS bandwidth allocation congured for a priority group in a DCB map, do not modify the existing DCB map
conguration. Instead, rst create a new DCB map with the desired PFC and ETS settings, and apply the new map to the interfaces to
override the previous DCB map settings. Then, delete the original dot1p priority-priority group mapping.
If you delete the dot1p priority-priority group mapping (no priority pgid command) before you apply the new DCB map, the
default PFC and ETS parameters are applied on the interfaces. This change may create a DCB mismatch with peer DCB devices and
interrupt network operation.
Data Center Bridging: Default Conguration
Before you congure PFC and ETS on a switch see the priority group setting taken into account the following default settings:
DCB is enabled.
PFC and ETS are globally enabled by default.
The default dot1p priority-queue assignments are applied as follows:
DellEMC(conf)#do show qos dot1p-queue-mapping
Dot1p Priority : 0 1 2 3 4 5 6 7
Queue : 0 0 0 1 2 3 3 3
DellEMC(conf)#
PFC is not applied on specic dot1p priorities.
ETS: Equal bandwidth is assigned to each port queue and each dot1p priority in a priority group.
To congure PFC and ETS parameters on an interface, you must specify the PFC mode, the ETS bandwidth allocation for a priority group,
and the 802.1p priority-to-priority group mapping in a DCB map. No default PFC and ETS settings are applied to Ethernet interfaces.
Conguring Priority-Based Flow Control
Priority-Based Flow Control (PFC) provides a ow control mechanism based on the 802.1p priorities in converged Ethernet trac received
on an interface and is enabled by default when you enable DCB.
As an enhancement to the existing Ethernet pause mechanism, PFC stops trac transmission for specied priorities (Class of Service
(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 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) trac that requires no-drop service, while retaining
packet-drop congestion management for Local Area Network (LAN) trac.
To congure PFC, follow these steps:
1 Create a DCB Map.
CONFIGURATION mode
dcb-map dcb-map-name
The dcb-map-name variable can have a maximum of 32 characters.
2 Create a PFC group.
CONFIGURATION mode
priority-group group-num {bandwidth bandwidth | strict-priority} pfc on
The range for priority group is from 0 to 7.
Data Center Bridging (DCB)
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