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The following illustration shows how PFC handles traffic congestion by pausing the transmission of incoming
traffic with dot1p priority 4.
Figure 28. Illustration of Traffic Congestion
The system supports loading two DCB_Config files:
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FCoE converged traffic with priority 3.
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iSCSI storage traffic with priority 4.
In the Dell Networking OS, PFC is implemented as follows:
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PFC is supported on specified 802.1p priority traffic (dot1p 0 to 7) and is configured per interface.
However, only 4 lossless queues are supported on an interface.
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PFC delay constraints place an upper limit on the transmit time of a queue after receiving a message to
pause a specified priority.
•
DCB is disabled on the switch
•
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
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During DCBx negotiation with a remote peer:
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DCBx communicates with the remote peer by LLDP TLV to determine current policies, such as PFC
support and ETS bandwidth allocation.
•
If DCBx negotiation is not successful (for example, a version or TLV mismatch), DCBx is disabled
and PFC or ETS cannot be enabled.
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PFC uses DCB MIB IEEE 802.1azd2.5 and PFC MIB IEEE 802.1bb-d2.2.
•
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 enable 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
link-level flow control.
•
Buffer space is allocated and de-allocated only when you configure a PFC priority on the port.
Data Center Bridging (DCB)
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Troubleshooting
