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Dell Configuration Guide for the S4048–ON System 9.14.2.6

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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 1. Illustration of Traffic Congestion. Illustration of Traffic Congestion
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 EMC 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.
  • PFC uses DCB MIB IEEE 802.1azd2.5 and PFC MIB IEEE 802.1bb-d2.2.
  • 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 enable DCB. If you have not loaded FCoE_DCB_Config and iSCSI_DCB_Config, DCB is disabled. When you enable DCB globally, you cannot simultaneously enable link-level flow control.
  • Buffer space is allocated and de-allocated only when you configure a PFC priority on the port.

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