Alcatel-Lucent 7450 Quality Of Service Manual page 181

The difference between the MBS size for the queue and the high priority reserve defines the threshold
where lower priority traffic will be discarded. The result is used on the queue to define a threshold
where lower priority packets are discarded, leaving the rest of the default MBS size for high priority
packets only. If the current MBS for the queue is 10MBytes, a value of 5 will result in a high priority
reserve on the queue of 576 KBytes. A value of 0 specifies that none of the MBS of the queue will be
reserved for high priority traffic. This does not affect RED slope operation for packets attempting to
be queued.
Modifying the current MBS for the queue through the mbs command will cause the default high-prio-
only function to be recalculated and applied to the queue. The high-prio-only command as defined for
the specific queue can be used to override the default high-prio-only setting as defined in the network
queue policy. Network queues also have a hi-low-prio-only drop tail which defaults to an additional
10% of the MBS value on top of the high-prio-only setting, however, the hi-low-prio-only is not
configurable for egress network queues in a network queue policy.
Network queues also have a hi-low-prio-only drop tail which defaults to an additional 10% of the
MBS value on top of the high-prio-only setting, however, the hi-low-prio-only is not configurable for
egress network queues in a network queue policy.
The no form of this command restores the default value.
Default 10%
Parameters
percent — The amount of queue buffer space, expressed as a decimal percentage of the MBS.
mbs
Syntax mbs percent
no mbs
Context config>qos>network-queue>queue
Description
The Maximum Burst Size (mbs) command specifies the relative amount of the buffer pool space for
the maximum buffers for a specific ingress network XMA or MDA forwarding class queue or egress
network port forwarding class queue. The value is entered as a percentage.
The MBS value is used to by a queue to determine whether it has exhausted its total allowed buffers
while enqueuing packets. Once the queue has exceeded its maximum amount of buffers, all packets
are discarded until the queue transmits a packet. A queue that has not exceeded its MBS size is not
guaranteed that a buffer will be available when needed or that the packet's RED slope will not force
the discard of the packet. Setting proper CBS parameters and controlling CBS oversubscription 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 the network queues.
The MBS size can sometimes be smaller than the CBS. This will result in a portion of the CBS for the
queue to be unused and should be avoided.
The no form of the command returns the MBS size for the queue to the default for the forwarding class.
Quality of Service Guide
Values 0 to 100, default
Network Queue QoS Policies
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