Administrator Guide

Priority-Based Flow Control
In a data center network, priority-based ow control (PFC) manages large bursts of one trac type in multiprotocol links so that it
does not aect other trac types and no frames are lost due to congestion.
When PFC detects congestion on a queue for a specied priority, it sends a pause frame for the 802.1p priority trac to the
transmitting device. In this way, PFC ensures that PFC-enabled priority trac is not dropped by the switch.
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 trac on a link (as performed by the traditional Ethernet pause mechanism), PFC pauses trac on a
link according to the 802.1p priority set on a trac type. You can create lossless ows for storage and server trac while allowing for
loss in case of LAN trac congestion on the same physical interface.
The following illustration shows how PFC handles trac congestion by pausing the transmission of incoming trac with dot1p
priority 3.
Figure 27. Priority-Based Flow Control
In the system, PFC is implemented as follows:
• PFC is supported on specied 802.1p priority trac (dot1p 0 to 7) and is congured per interface. However, only two lossless
queues are supported on an interface: one for Fibre Channel over Ethernet (FCoE) converged trac and one for Internet Small
Computer System Interface (iSCSI) storage trac. Congure 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 specied priority.
• By default, PFC is enabled on an interface with no dot1p priorities congured. You can congure the PFC priorities if the switch
negotiates with a remote peer using DCBX.
• During DCBX negotiation with a remote peer:
– If the negotiation succeeds and the port is in DCBX Willing mode to receive a peer conguration, PFC parameters from the
peer are used to congured 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-congured PFC input policies are applied. If no PFC input
policy has been previously applied, the PFC default setting is used (no priorities congured). 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.
• PFC supports buering 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.
Enhanced Transmission Selection
Enhanced transmission selection (ETS) supports optimized bandwidth allocation between trac types in multiprotocol (Ethernet,
FCoE, SCSI) links.
ETS allows you to divide trac according to its 802.1p priority into dierent priority groups (trac classes) and congure bandwidth
allocation and queue scheduling for each group to ensure that each trac type is correctly prioritized and receives its required
bandwidth. For example, you can prioritize low-latency storage or server cluster trac in a trac class to receive more bandwidth
and restrict best-eort LAN trac assigned to a dierent trac class.
Although you can congure strict-priority queue scheduling for a priority group, ETS introduces exibility that allows the bandwidth
allocated to each priority group to be dynamically managed according to the amount of LAN, storage, and server trac in a ow.
Data Center Bridging (DCB)
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