Administrator Guide
Priority-Based Flow Control
In a data center network, priority-based ow control (PFC) manages large bursts of one trac type in multiprotocol links so that it
does not aect other trac types and no frames are lost due to congestion.
When PFC detects congestion on a queue for a specied priority, it sends a pause frame for the 802.1p priority trac to the
transmitting device. In this way, PFC ensures that large amounts of queued LAN trac do not cause storage trac to be dropped,
and that storage trac does not result in high latency for high-performance computing (HPC) trac between servers.
PFC enhances the existing 802.3x pause and 802.1p priority capabilities to enable ow control based on 802.1p priorities (classes of
service). Instead of stopping all trac on a link (as performed by the traditional Ethernet pause mechanism), PFC pauses trac on a
link according to the 802.1p priority set on a trac type. You can create lossless ows for storage and server trac while allowing for
loss in case of LAN trac congestion on the same physical interface.
The following illustration shows how PFC handles trac congestion by pausing the transmission of incoming trac with dot1p
priority 3.
Figure 1. Priority-Based Flow Control
In the system, PFC is implemented as follows:
• PFC is supported on specied 802.1p priority trac (dot1p 0 to 7) and is congured per interface. However, only two lossless
queues are supported on an interface: one for Fibre Channel over Ethernet (FCoE) converged trac and one for Internet Small
Computer System Interface (iSCSI) storage trac. Congure the same lossless queues on all ports.
• A dynamic threshold handles intermittent trac bursts and varies based on the number of PFC priorities contending for buers,
while a static threshold places an upper limit on the transmit time of a queue after receiving a message to pause a specied
priority. PFC trac is paused only after surpassing both static and dynamic thresholds for the priority specied 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 ow control and link-level ow control is disabled.
• Buer space is allocated and de-allocated only when you congure 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 specied priority.
• By default, PFC is enabled on an interface with no dot1p priorities congured. You can congure 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 conguration, PFC parameters from the
peer are used to congured PFC priorities on the port. If you enable the link-level ow control mechanism on the interface,
DCBX negotiation with a peer is not performed.
– If the negotiation fails and PFC is enabled on the port, any user-congured PFC input policies are applied. If no PFC dcb-map
has been previously applied, the PFC default setting is used (no priorities congured). If you do not enable PFC on an
interface, you can enable the 802.3x link-level pause function. By default, the link-level pause is disabled, when you disable
DCBx and PFC. If no PFC dcb-map has been applied on the interface, the default PFC settings are used.
• PFC supports buering to receive data that continues to arrive on an interface while the remote system reacts to the PFC
operation.
Data Center Bridging (DCB)
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