Configuring Data Center Bridging Features 833
At the first level of egress scheduling, each of the configured attributes of a
CoS queue, namely scheduler algorithm, min-bandwidth and drop
mechanism, are honored, and the packet is either dropped or forwarded to
next level. Only frames selected by the first level scheduler are forwarded to
the second level.
Strict priority traffic classes are serviced first in order of traffic class number. A
strict priority traffic class is one that is configured as strict priority or has a
traffic class group weight of 0 (unlimited bandwidth). Within a strict priority
traffic class, frames are serviced (as determined by the traffic class number) in
CoS queue priority order from the highest (6) to the lowest (0).
Each traffic class has an internal weight equal to the traffic class number plus
one. After the strict priority traffic classes have been serviced, the remaining
traffic classes are serviced according to their internal weight. For example, if
CoS queues 0, 1, and 2 have an equal offered load toward a congested output
port, CoS queue 2 will receive 3/6 of the bandwidth, CoS queue 1 will receive
2/6 of the bandwidth, and CoS queue 0 will receive 1/6 of the bandwidth.
The minimum bandwidth setting can be used to override the strict priority
and weighted settings. The highest numbered strict priority queue will
receive no more bandwidth than 100 percent minus the sum of the minimum
bandwidth percentages assigned to the other queues. If used, it is
recommended that minimum bandwidth percentages only be set high
enough to ensure a minimum level of service for any queue; i.e., the sum of
the minimum bandwidth percentages is a fraction of 100%. This ensures that
the system can respond to bursts in traffic. Setting the minimum bandwidths
such that they sum to 100% effectively sets the scheduler such that no queue
can exceed its minimum bandwidth percentage when congestion occurs.
Each CoS queue in the first level scheduler is mapped to one of the three
traffic class groups in the second level scheduler. There, frames are serviced
using the TCG configuration. The minimum bandwidth guarantee is first
calculated across the TCGs. Strict priority TCGs are scheduled first but have
their bandwidth reduced by the minimum bandwidth guarantees configured
on other TCGs. Strict priority TCGs are scheduled from highest numbered
TCG to lowest. When all TCGs have met their minimum bandwidth limits
NOTE: CoS queue 7 is reserved for internal traffic. Non-strict priority CoS queues
are serviced with WDRR scheduling using the bandwidth available after strict
priority traffic is serviced.
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