Reference Guide

284 | Data Center Bridging (DCB)
www.dell.com | support.dell.com
Configuring PFC and ETS in a DCB Map
An S5000 switch supports the use of a DCB map in which you configure priority-based flow control and
enhanced transmission selection settings. To configure PFC and ETS parameters, you must apply a DCB
map on an S5000 interface (see Data Center Bridging: Default Configuration).
PFC Configuration Notes
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 (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 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 an S5000 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 DCB Map: Configuration Procedure and Configuring PFC without a DCB Map.
When you configure PFC in a DCB map:
• 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 operation on ingress port traffic. To
achieve complete lossless handling of traffic, configure PFC priorities on all DCB egress ports.
• 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 (
flowcontrol rx on tx on command; see
Using Ethernet Pause Frames for Flow Control). 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).
Note: DCB maps are supported only on physical Ethernet interfaces.