Alcatel-Lucent 7450 Quality Of Service Manual page 179

Description The Committed Burst Size (cbs) command specifies the relative amount of reserved 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 CBS for a queue is used to determine whether it has exhausted its reserved buffers while
enqueuing packets. Once the queue has exceeded the amount of buffers considered in reserve for this
queue, it must contend with other queues for the available shared buffer space within the buffer pool.
Access to this shared pool space is controlled through Random Early Detection (RED) slope
application.
Two RED slopes are maintained in each buffer pool. A high priority slope is used by in-profile packets.
A low priority slope is used by out-of-profile packets. At egress, there is an additional RED slope
maintained in each buffer pool, the exceed slope, which is used by exceed-profile packets. All
Network-Control and Management packets are considered in-profile. Assured packets are handled by
their in-profile and out-of-profile markings. All Best-Effort packets are considered out-of-profile.
Premium queues should be configured such that the CBS percent is sufficient to prevent shared
buffering of packets. This is generally taken care of by the CIR scheduling of Premium queues and the
overall small amount of traffic on the class. Premium queues in a properly designed system will drain
before all others, limiting their buffer utilization.
The RED slopes will detect congestion conditions and work to discard packets and slow down random
TCP session flows through the queue. The RED slope definitions can be defined, modified or disabled
through the network-queue policy assigned to the XMA or MDA for the network ingress buffer pool
or assigned to the network port for network egress buffer pools.
The resultant CBS size can be larger than the MBS. This will result in a portion of the CBS for the
queue to be unused and should be avoided.
The no form of this command returns the CBS size for the queue to the default for the forwarding class.
Default
The cbs forwarding class defaults are listed in the
Quality of Service Guide
Table 26: cbs forwarding class defaults
Forwarding Class
Network-Control
High-1
Expedited
High-2
Low-1
Assured
Low-2
Best-Effort
Table 26.
Fowarding Class Label
nc
h1
ef
h2
l1
af
l2
be
Network Queue QoS Policies
Default CBS
3
3
1
1
3
1
3
1
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