Quality of Service Guide QoS and QoS Policies
Edition: 01 3HE 11014 AAAC TQZZA 33
3.1.6.2 CBS and MBS Configuration
On the access side, CBS is configured in bytes and MBS in bytes or kilobytes using
the CLI. See, for example, the config>qos>sap-ingress/egress>queue>cbs and
mbs configuration commands.
On the network side, CBS and MBS values are expressed as a percentage of the
total number of available buffers. If the buffer space is further segregated into pools
(for example, ingress and egress, access and network, or a combination of these),
the CBS and MBS values are expressed as a percentage of the applicable buffer
pool. See the config>qos>network-queue>queue>cbs and mbs configuration
commands.
The configured CBS and MBS values are converted to the number of buffers by
dividing the CBS or MBS value by a fixed buffer size of 512 bytes or 2304 bytes,
depending on the type of adapter card or platform. The number of buffers can be
displayed for an adapter card using the show pools command.
3.1.6.2.1 Buffer Allocation for Multicast Traffic
When a packet is being multicast to two or more interfaces on the egress adapter
card or block of fixed ports, or when a packet at port ingress is mirrored, one extra
buffer per packet is used.
In previous releases, this extra buffer was not added to the queue count. When
checking CBS during multicast traffic enqueuing, the CBS was divided by two to
prevent buffer overconsumption by the extra buffers. As a result, during multicast
traffic enqueuing, the CBS buffer limit for the queue was considered reached when
half of the available buffers were in use.
In Release 8.0 of the 7705 SAR, the CBS is no longer divided by two. Instead, the
extra buffers are added to the queue count when enqueuing, and are removed from
the queue count when the multicast traffic exits the queue. The full CBS value is
used, and the extra buffer allocation is visible in buffer allocation displays.
When upgrading to Release 8.0 of the 7705 SAR, ensure that previous CBS and
MBS configurations take extra buffers for multicast packets into account, and that
maximum delay budgets are not affected adversely.
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