Service Manual

NOTE: Dell Networking OS Behavior: By default, no lossless queues are configured on a port.
A limit of two lossless queues is supported on a port. If the amount of priority traffic that you configure to
be paused exceeds the two lossless queues, an error message displays.
Configuring PFC in a DCB Map
An S4048–ON switch supports the use of a DCB map in which you configure priority-based flow control
(PFC) setting. To configure PFC parameters, you must apply a DCB map on an S4048–ON interface. This
functionality is supported on the
S4048–ON platform.
PFC Configuration Notes
PFC provides flow control based on the 802.1p priorities in a converged Ethernet traffic 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 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 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.
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 configured. When a DCB input policy with PFC profile is
configured or unconfigured on an interface or a range of interfaces not receiving any traffic, interfaces
with PFC settings that receive appropriate PFC-enabled traffic (unicast, mixed-frame-size traffic) display
incremental values in the CRC and discards counters. (These ingress interfaces receiving pfc-enabled
traffic have an egress interface that has a compatible PFC configuration).
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.
Data Center Bridging (DCB)
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