Dell BigIron RX Series, Configuration Manual

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BigIron RX Series Configuration Guide 487
53-1002253-01
Determining packet drop priority using WRED
18
Determining packet drop priority using WRED
You can configure a
device
to monitor traffic congestion and drop packets according to a WRED
(Weighted Random Early Detection) algorithm. This algorithm enables the system to detect the
onset of congestion and take corrective action. In practice, WRED causes a Switch to start dropping
packets as traffic in the switch starts to back up. WRED provides various control points that can be
configured to change a system's reaction to congestion. The following variables are used when
calculating whether to drop or forward packets:
•
Statistical Average-Q-Size – The statistical average size of the queue calculated over time on
the switch.
•
Current-Q-Size – The current size of the queue as calculated on the switch.
•
Wq – This variable specifies the weights that should be given to the current queue size and the
statistical average-q-size when calculating the size for WRED calculations.
•
Max-Instantaneous-Q-Size – The maximum size up to which a queue is allowed to grow.
Packets that cause the queue to grow beyond this point are unconditionally dropped. This
variable is user configured.
•
Min-Average-Q-Size – The average queue size below which all packets are accepted. This
variable is user configured.
•
Max-Average-Q-Size – The average queue size above which all packets are dropped. This
variable is user configured.
•
Pmax – The maximum drop probability when queue-size is at Max-Average-Q-Size. This variable
is user configured.
•
Pkt-Size-Max – The packet size to which the current packet's size is compared as shown in the
algorithm below. This variable is user configured.
Mark The marking type enabled on the interface. The marking type can be
any of the following:
•
COS – CoS marking is enabled.
•
DSCP – DSCP marking is enabled.
•
No – Marking is not enabled.
Trust-Level The trust level enabled on the interface. The trust level can be one of
the following:
•
DSCP
•
L2 CoS
CoS-DSCP map
COS The CoS (802.1p) values.
dscp The DSCP values to which the device maps the CoS values above.
DSCP-priority map
d1 and d2 The DSCP -> forwarding priority mappings that are currently in effect.
DSCP-DSCP map
d1 and d2 The DSCP -> DSCP mappings that are currently in effect.
CoS-priority map The CoS (802.1p) forwarding priority mapping that is currently in effect.
TABLE 95 ToS-based QoS configuration information (Continued)
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