QoS Queue Group Template Command Reference
448 Quality of Service Guide
Parameters percent-of-mbs — The percent-of-mbs parameter is required when specifying high-prio-only and
is expressed as a percentage.
Values 0 to 100
Default 10
mbs
Syntax mbs size [bytes | kilobytes]
no mbs
Context config>qos>qgrps>egr>qgrp>queue
config>qos>qgrpid>ing>qgrp>queue
config>qos>qgrpid>egr>qgrp>policer
config>qos>qgrpid>ing>qgrp>policer
Description The Maximum Burst Size (MBS) command specifies the default maximum buffer size for the template
queue. The value is given in bytes or kilobytes.
The MBS value is used by a queue to determine whether it has exhausted all of its buffers while
enqueuing packets. Once the queue has exceeded the amount of buffers allowed by MBS, all packets
are discarded until packets have been drained from the queue.
The port Ethernet access ingress and egress queue group context for mbs provides a mechanism for
overriding the default maximum size for the queue.
The sum of the MBS for all queues on an ingress access port can oversubscribe the total amount of
buffering available. When congestion occurs and buffers become scarce, access to buffers is controlled
by the RED slope a packet is associated with. A queue that has not exceeded its MBS size is not
guaranteed that a buffer will be available when needed or that the packets RED slope will not force the
discard of the packet. Setting proper CBS parameters and controlling CBS over-subscription is one
major safeguard to queue starvation (when a queue does not receive its fair share of buffers). Another
is properly setting the RED slope parameters for the needs of services on this port or channel.
When configured on an egress queue group queue, this command and the queue-delay command are
mutually exclusive. In order to change between the mbs and queue-delay parameters, the current
parameter must be removed before adding the new parameter, that is, changing from mbs to queue-
delay requires a no mbs before the queue-delay is configured and changing from queue-delay to mbs
requires a no queue-delay before the mbs is configured. If queue-delay is configured for an egress
queue group queue, it is not possible to override the MBS for that queue.
For policers, this command is used to configure the policer’s PIR leaky bucket’s high priority violate
threshold. The high-prio-only command is applied to the MBS value to derive the bucket’s low
priority violate threshold.
For ingress, trusted in-profile packets and untrusted high priority packets use the policer’s high priority
violate threshold while trusted out-of-profile and untrusted low priority packets use the policer's low
priority violate threshold. At egress, in-profile packets use the policer’s high priority violate threshold
and out-of-profile packets use the policer's low priority violate threshold.
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