Users Guide

Step Task Command Command Mode
4
Return to interface conguration mode.
exit
DCB MAP
5
Apply the DCB map, created to disable the PFC
operation, on the interface
dcb-map {name | default}
INTERFACE
6
Congure the port queues that still function as no-drop
queues for lossless trac. For the dot1p-queue
assignments.
The maximum number of lossless queues globally
supported on a port is 2.
You cannot congure PFC no-drop queues on an
interface on which a DCB map with PFC enabled has
been applied, or which is already congured for PFC
using the pfc priority command.
Range: 0-3. Separate queue values with a comma;
specify a priority range with a dash; for example: pfc no-
drop queues 1,3 or pfc no-drop queues 2-3 Default: No
lossless queues are congured.
pfc no-drop
queuesqueue-range
INTERFACE
Priority-Based Flow Control Using Dynamic Buer Method
Priority-based ow control using dynamic buer spaces is supported on the S4820T 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
resume thresholds can also be congured dynamically. You can congure a buer size, pause threshold, ingress shared threshold
weight, and resume threshold to control and manage the total amount of buers that are to be used in your network environment.
Buer Sizes for Lossless or PFC Packets
You can congure up to a maximum of 4 lossless (PFC) queues. By conguring 4 lossless queues, you can congure 4 dierent
priorities and assign a particular priority to each application that your network is used to process. For example, you can assign a
higher priority for time-sensitive applications and a lower priority for other services, such as le transfers. You can congure the
amount of buer space to be allocated for each priority and the pause or resume thresholds for the buer. This method of
conguration enables you to eectively manage and administer the behavior of lossless queues.
Data Center Bridging (DCB)
249