User manual

C HAPTER 4 | Configuring the Switch
Quality of Service
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CONFIGURING WRED Use the Storm Control Configuration page to control traffic congestion on
its output queues using Weighted Random Early Detection (WRED). This
method controls the average queue size by randomly dropping packets at a
moderate rate as the network load moves above a specified minimum
threshold, and then at a more aggressive rate when it reaches the
maximum threshold. If the source is using TCP, it will automatically
decrease its transmission rate once it notes that packets are being
dropped. RED and TCP work together to cause hosts to adjust their
transmissions to a rate the network can handle.
WRED provides preferential treatment of higher priority frames when traffic
builds up within a queue. A frame's DP level is used as input to WRED. A
higher DP level assigned to a frame results in a higher probability that the
frame is dropped during times of congestion.
N OTE : Neither Novell NetWare nor AppleTalk respond robustly to packet
loss, either ignoring the dropped packets, or resending them at the same
rate. If a significant percentage of the network’s traffic employs these
protocols, it is not advisable to enable RED.
PATH
Configuration, QoS, WRED
PARAMETERS
These parameters are displayed:
◆ Queue - The ID of the priority queue. (Range: 0-7, where 7 is the
highest priority queue)
◆ Enable - Controls whether RED is enabled for this queue.
◆ Min. Threshold - Sets the lower RED threshold as a percentage of
queue capacity. If the average queue loading is below this threshold,
the drop probability is zero. (Range: 0-100)
◆ Max. DP 1-3 - Controls the drop probability for frames marked with
Drop Precedence Level 1-3 when the average queue loading is 100%.
(Range: 0-100)
Every incoming frame is classified to a Drop Precedence Level (DP
level), which is used throughout the device for providing congestion
control guarantees for the frame according to value configured for that
specific DP level. Inbound traffic is marked for drop precedence using a
three-color priority system. Drop precedence is normally set from a
lower to higher level for green (DP1), yellow (DP2), and then red
(DP3). The internal DSCP map is used to mark inbound traffic based on
priority bits in the VLAN tag or Layer 2 traffic, or the IP Precedence or
DSCP value for Layer 3 traffic.