Dell PowerEdge FX2/FX2s - Manual (Page 28)

NOTE: In Dell Networking OS version 9.4.0.x, only the PFC, ETS, and DCBx features are supported 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 large amounts of queued LAN
traffic do not cause storage traffic to be dropped, and that storage traffic does not result in high latency
for high-performance computing (HPC) traffic between servers.
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 3.
Figure 1. Priority-Based Flow Control
In the system, 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.
• A dynamic threshold handles intermittent traffic bursts and varies based on the number of PFC
priorities contending for buffers, while a static threshold places an upper limit on the transmit time of
a queue after receiving a message to pause a specified priority. PFC traffic is paused only after
surpassing both static and dynamic thresholds for the priority specified for the port.
• By default, PFC is enabled when you enabled DCB. When you enable DCB globally, you cannot
simultaneously enable TX and RX on the interface for flow control and link-level flow control is
disabled.
• Buffer space is allocated and de-allocated only when you configure a PFC priority on the port.
• PFC delay constraints place an upper limit on the transmit time of a queue after receiving a message
to pause a specified priority.
28 Data Center Bridging (DCB)