Administrator Guide

To ensure lossless delivery and latency-sensitive scheduling of storage and service trac and I/O convergence of LAN, storage, and
server trac over a unied fabric, IEEE data center bridging adds the following extensions to a classical Ethernet network:
• 802.1Qbb — Priority-based Flow Control (PFC)
• 802.1Qaz — Enhanced Transmission Selection (ETS)
• 802.1Qau — Congestion Notication
• Data Center Bridging Exchange (DCBx) protocol
NOTE: In the Dell Networking OS version 8.3.12.0, only the PFC, ETS, and DCBx features are supported in data center
bridging.
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 141. 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.
FC Flex IO Modules
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