Reference Guide

Data Center Bridging (DCB) | 283
FTOS Behavior:
Traffic in priority groups is assigned to strict-queue or WERR scheduling in an DCB Output policy
and is managed using the ETS bandwidth-assignment algorithm. FTOS deqeues all frames of
strict-priority traffic before servicing any other queues. A queue with strict-priority traffic can starve
other queues in the same port.
ETS-assigned bandwidth allocation and scheduling apply only to data queues, not to control queues.
FTOS supports hierarchical scheduling on an interface. FTOS control traffic is redirected to control
queues as higher priority traffic with strict priority scheduling. After control queues drain out, the
remaining data traffic is scheduled to queues according to the bandwidth and scheduler configuration
in the DCB Output policy. The available bandwidth calculated by the ETS algorithm is equal to the
link
bandwidth after scheduling non-ETS higher-priority traffic.
The configuration of bandwidth allocation and strict-queue scheduling is not supported at the same
time for a priority group. If both are configured, the configured bandwidth allocation is ignored for
priority-group traffic when you apply the output policy on an interface (refer to Applying an DCB
Output Policy for a Priority Group to an Interface).
Bandwidth assignment in a dot.1p priority-queue: By default, equal bandwidth is assigned to each
port queue and each dot1p priority in a priority group. Use the bandwidth-percentage command to
configure bandwidth amounts in associated dot1p queues. When specified bandwidth is assigned to
some port queues and not to others, the remaining bandwidth (100% minus assigned bandwidth
amount) is equally distributed to unassigned non-strict priority queues in the priority group. The sum
of the allocated bandwidth to all queues in a priority group should be 100% of the bandwidth on the
link.
Bandwidth assignment in a priority group: By default, equal bandwidth is assigned to each
priority group in the DCB Output policy applied to an egress port if you did not configure bandwidth
allocation.The sum of configured bandwidth allocation to dot1p priority traffic in all ETS priority
groups must be 100%. You must allocate at least 1% of the total bandwidth to each priority group and
queue. If you assign bandwidth to some priority groups but not to others, the remaining bandwidth
(100% minus assigned bandwidth amount) is equally distributed to non-strict-priority groups which
have no configured scheduler.
Scheduling of priority traffic: dot1p priority traffic on the switch is scheduled according to the
current dot1p-queue mapping. dot1p priorities within the same queue should have the same traffic
properties and scheduling method.
DCB Output-policy error: If an error occurs in an DCB Output-policy configuration, the
configuration is ignored and the scheduler and bandwidth allocation settings are reset to the ETS
default values (all priorities are in the same ETS priority group and bandwidth is allocated equally to
each priority).
If an error occurs when a port receives a peer’s ETS configuration, the port’s configuration is reset to
the previously configured DCB Output policy. If no DCB Output policy was previously applied, the
port is reset to the default ETS parameters.