xStack Gigabit Layer 3 Switch Command Line Interface Manual
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OMMANDS
The xStack family of switches supports 802.1p priority queuing. These switches has eight classes of service for each port on the
Switch, one of which is internal and unconfigurable to the user. These hardware classes of service are numbered from 6 (Class
6) — the highest hardware class of service — to 0 (Class 0) — the lowest hardware class of service. The eight priority tags
specified in IEEE 802.1p (p0 to p7) are mapped to the Switch’s hardware classes of service as follows:
•
Priority 0 is assigned to the Switch’s Q2 class.
•
Priority 1 is assigned to the Switch’s Q0 class.
•
Priority 2 is assigned to the Switch’s Q1 class.
•
Priority 3 is assigned to the Switch’s Q3 class.
•
Priority 4 is assigned to the Switch’s Q4 class.
•
Priority 5 is assigned to the Switch’s Q5 class.
•
Priority 6 is assigned to the Switch’s Q6 class.
•
Priority 7 is assigned to the Switch’s Q6 class.
Priority scheduling is implemented using two types of methods, strict priority and weight fair priority. If no changes are made to
the QoS priority scheduling settings the method used is strict priority.
NOTICE:
The Switch contains eight classes of service for
each port on the Switch. One of these classes is reserved
for internal use on the Switch and therefore is not
configurable. All references in the following section
regarding classes of service will refer to only the seven
classes of service that may be used and configured by the
Switch’s Administrator.
For strict priority-based scheduling, packets residing in the higher hardware classes of service are transmitted first. Only when
these classes are empty, are packets of lower hardware class allowed to be transmitted. Higher priority tagged packets always
receive precedence regardless of the amount of lower priority tagged packets in the buffer and regardless of the time elapsed
since any lower priority tagged packets have been transmitted. By default, the Switch is configured to empty the buffer using
strict priority.
NOTICE:
The default QoS scheduling arrangement is a
strict priority schedule. To customize scheduling to set up
weight fair queue clearing, the MAX. Packets values need to
be changed using the
config scheduling
command. See
config scheduling
below.
To use implement weight fair priority, the Switch’s seven hardware classes of service can be configured to reduce the buffer in a
weighted round-robin (
WRR
) fashion - beginning with the highest hardware class of service, and proceeding to the lowest
hardware class of service before returning to the highest hardware class of service.
The weighted-priority based scheduling alleviates the main disadvantage of strict priority-based scheduling
−
in that lower
priority classes of service get starved of bandwidth
−
by providing a minimum bandwidth to all queues for transmission. This is
accomplished by configuring the maximum number of packets allowed to be transmitted from a given priority class of service
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