Priority-Based Flow Control - Dell S4048T Configuration Manual

• 802.1Qau — Congestion Notification
• Data Center Bridging Exchange (DCBx) protocol
NOTE: Dell Networking OS supports only the PFC, ETS, and DCBx features in data center bridging.

Priority-Based Flow Control

In a data center network, priority-based flow control (PFC) manages large bursts of one traffic type in
multiprotocol links so that it does not affect other traffic types and no frames are lost due to congestion.
When PFC detects congestion on a queue for a specified priority, it sends a pause frame for the 802.1p
priority traffic to the transmitting device. In this way, PFC ensures that PFC-enabled priority traffic is not
dropped by the switch.
PFC enhances the existing 802.3x pause and 802.1p priority capabilities to enable flow control based on
802.1p priorities (classes of service). Instead of stopping all traffic on a link (as performed by the traditional
Ethernet pause mechanism), PFC pauses traffic on a link according to the 802.1p priority set on a traffic type.
You can create lossless flows for storage and server traffic while allowing for loss in case of LAN traffic
congestion on the same physical interface.
The following illustration shows how PFC handles traffic congestion by pausing the transmission of incoming
traffic with dot1p priority 4.
Figure 32. Illustration of Traffic Congestion
The system supports loading two DCB_Config files:
• FCoE converged traffic with priority 3.
• iSCSI storage traffic with priority 4.
In the Dell Networking OS, PFC is implemented as follows:
• PFC is supported on specified 802.1p priority traffic (dot1p 0 to 7) and is configured per interface.
However, only two lossless queues are supported on an interface: one for Fibre Channel over Ethernet
(FCoE) converged traffic and one for Internet Small Computer System Interface (iSCSI) storage traffic.
Configure the same lossless queues on all ports.
• PFC supports buffering to receive data that continues to arrive on an interface while the remote system
reacts to the PFC operation.
Data Center Bridging (DCB) 295