Dell EMC Networking OS Configuration Guide for the S5048F–ON System 9.14.2.4

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.

The system supports loading two DCB_Config files:

• FCoE converged traffic with priority 3.
• iSCSI storage traffic with priority 4.

• FCoE converged traffic with priority 3.

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 one lossless queues arequeue is supported on an interface: one for Fibre Channel over Ethernet (FCoE) converged traffic and one for Internet Small Computer System Interface (iSCSI) storage trafficone for Fibre Channel over Ethernet (FCoE) converged traffic. Configure the same lossless queues on all ports.
• PFC delay constraints place an upper limit on the transmit time of a queue after receiving a message to pause a specified priority.
• By default, PFC is enabled on an interface with no dot1p priorities configured. You can configure the PFC priorities if the switch negotiates with a remote peer using DCBx
• During DCBx negotiation with a remote peer:
• DCBx communicates with the remote peer by LLDP TLV to determine current policies, such as PFC support and ETS bandwidth allocation.
• If DCBx negotiation is not successful (for example, a version or TLV mismatch), DCBx is disabled and PFC or ETS cannot be enabled.
• PFC supports buffering to receive data that continues to arrive on an interface while the remote system reacts to the PFC operation.
• 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.