Administrator Guide

Hierarchical Scheduling in ETS Output Policies
ETS supports up to three levels of hierarchical scheduling.
For example, you can apply ETS output policies with the following congurations:
Priority group 1 Assigns trac to one priority queue with 20% of the link bandwidth and strict-priority scheduling.
Priority group 2 Assigns trac to one priority queue with 30% of the link bandwidth.
Priority group 3 Assigns trac to two priority queues with 50% of the link bandwidth and strict-priority scheduling.
In this example, the congured ETS bandwidth allocation and scheduler behavior is as follows:
Therefore, in this example, scheduling trac to priority group 1 (mapped to one strict-priority queue) takes precedence over scheduling
trac to priority group 3 (mapped to two strict-priority queues).
Unused bandwidth
usage:
Normally, if there is no trac or unused bandwidth for a priority group, the bandwidth allocated to the group is
distributed to the other priority groups according to the bandwidth percentage allocated to each group. However,
when three priority groups with dierent bandwidth allocations are used on an interface:
• If priority group 3 has free bandwidth, it is distributed as follows: 20% of the free bandwidth to priority group 1
and 30% of the free bandwidth to priority group 2.
• If priority group 1 or 2 has free bandwidth, (20 + 30)% of the free bandwidth is distributed to priority group 3.
Priority groups 1 and 2 retain whatever free bandwidth remains up to the (20+ 30)%.
Strict-priority
groups:
If two priority groups have strict-priority scheduling, trac assigned from the priority group with the higher
priority-queue number is scheduled rst. However, when three priority groups are used and two groups have strict-
priority scheduling (such as groups 1 and 3 in the example), the strict priority group whose trac is mapped to one
queue takes precedence over the strict priority group whose trac is mapped to two queues.
Priority-Based Flow Control Using Dynamic Buer
Method
Priority-based ow control using dynamic buer spaces is supported on the switch.
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.
Data Center Bridging (DCB)
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