Users Guide

The following prerequisites and restrictions apply when you congure PFC in a DCB map:
• You can enable PFC on a maximum of two priority queues on an interface. Enabling PFC for dot1p priorities congures the
corresponding port queue as lossless.
• You cannot enable PFC and link-level ow control at the same time on an interface.
Applying a DCB Map on a Port
When you apply a DCB map with PFC enabled on a switch interface, a memory buer for PFC-enabled priority trac is
automatically allocated. The buer size is allocated according to the number of PFC-enabled priorities in the assigned map.
To apply a DCB map to an Ethernet port, follow these steps:
Table 17. DCB Map to an Ethernet Port
Step Task Command Command Mode
1
Enter interface conguration mode on an Ethernet port.
interface {tengigabitEthernet
slot/port |
fortygigabitEthernet slot/
port}
CONFIGURATION
2
Apply the DCB map on the Ethernet port to congure it
with the PFC and ETS settings in the map; for example:
Dell# interface tengigabitEthernet 1/1
Dell(cong-if-te-1/1)# dcb-map SAN_A_dcb_map1 Repeat
Steps 1 and 2 to apply a DCB map to more than one port.
You cannot apply a DCB map on an interface that has been
already congured for PFC using thepfc priority
command or which is already congured for lossless
queues (pfc no-drop queues command).
dcb-map name
INTERFACE
Priority-Based Flow Control Using Dynamic Buer Method
Priority-based ow control using dynamic buer spaces is supported on the S4810 platform.
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.
Pause and Resume of Trac
The pause message is used by the sending device to inform the receiving device about a congested, heavily-loaded trac state that
has been identied. When the interface of a sending device transmits a pause frame, the recipient acknowledges this frame by
temporarily halting the transmission of data packets. The sending device requests the recipient to restart the transmission of data
trac when the congestion eases and reduces. The time period that is specied in the pause frame denes the duration for which
the ow of data packets is halted. When the time period elapses, the transmission restarts.
When a device sends a pause frame to another device, the time for which the sending of packets from the other device must be
stopped is contained in the pause frame. The device that sent the pause frame empties the buer to be less than the threshold
value and restarts the acceptance of data packets.
Dynamic ingress buering enables the sending of pause frames at dierent thresholds based on the number of ports that experience
congestion at a time. This behavior impacts the total buer size used by a particular lossless priority on an interface. The pause and
Data Center Bridging (DCB)
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