Reference Guide

Data Center Bridging (DCB) | 49
Figure 5-1. Priority-Based Flow Control
PFC is implemented as follows in the Dell Networking operating software (FTOS):
• PFC supports buffering to receive data that continues to arrive on an interface while the remote system
reacts to the PFC operation.
• PFC uses the DCB MIB IEEE802.1azd2.5 and the PFC MIB IEEE802.1bb-d2.2.
• 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 FCoE converged traffic and
one for 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. If you have not loaded FCoE_DCB_Config and
iSCSI_DCB_Config, DCB is disabled. 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.
• 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 link layer discovery protocol (LLDP) type, length,
value (TLV) to determine current policies, such as PFC support and enhanced transmission
selection (ETS) BW allocation.
• If the negotiation succeeds and the port is in DCBx Willing mode to receive a peer configuration,
PFC parameters from the peer are used to configured PFC priorities on the port. If you enable the
link-level flow control mechanism on the interface, DCBx negotiation with a peer is not
performed.
• If the negotiation fails and you enable PFC on the port, any user-configured PFC input policies are
applied. If no PFC input policy has been previously applied, the PFC default setting is used (no
priorities configured). If you do not enable PFC on an interface, you can enable the 802.3x
link-level pause function. By default, the link-level flow pause is disabled when you disable DCBx
and PFC. If no PFC input policy has been applied on the interface, the default PFC settings are
used.
• PFC supports buffering to receive data that continues to arrive on an interface while the remote system
reacts to the PFC operation.