Concept Guide

Conguration Notes: PFC and ETS in a DCB Map
The switch supports the use of a DCB map in which you congure priority-based ow control (PFC) and enhanced transmission selection
(ETS) settings. To congure PFC and ETS parameters, you must apply a DCB map on the 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
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 the switch, PFC is enabled 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.
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, when DCB is enabled.
• 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 with more than the maximum lossless queues congured, 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.
• Congure all the backplane ports of the linecards and RPM0 and RPM1 with same dcb-map conguration.
dcb-map linecard 0 backplane all name
dcb-map linecard all backplane all
name
• Dell 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 four PFC-enabled, lossless queues are supported on an interface.
Otherwise, the reconguration of a default dot1p-queue assignment is rejected.
• To ensure complete no-drop service, apply the same PFC parameters on all PFC-enabled peers.
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Data Center Bridging (DCB)