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• 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 flow control (PFC) manages large bursts of one traffic type in
multiprotocol links so that it does not affect other traffic types and no frames are lost due to congestion.
When PFC detects congestion on a queue for a specified priority, it sends a pause frame for the 802.1p
priority traffic to the transmitting device. In this way, PFC ensures that PFC-enabled priority traffic is not
dropped by the switch.
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 3.
Figure 139. Priority-Based Flow Control
In the system, 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 two lossless queues are supported on an interface: one for Fibre Channel over
Ethernet (FCoE) converged traffic and one for Internet Small Computer System Interface (iSCSI)
storage 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:
– 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 PFC is enabled 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
FC Flex IO Modules
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