Congestion Avoidance - 3Com 4500 PWR 26-Port Configuration Manual

Figure 45-8 Diagram for WRR queuing
WRR queue-scheduling algorithm schedules all the queues in turn and every queue can be assured of
a certain service time.
In a typical 3Com switch there are eight output queues on ea
each q ueue, for example: w7, w6, w5, w4, w3, w2, w1, and w0 respectively for queue 7 through queue
0. A weight value indicates the proportion of resources available for a queue. On a 100-Mbps port,
configure the weight value of WRR queue-scheduling algorithm to 5, 5, 3, 3, 1, 1, 1, and 1
(corresponding to w7, w6, w5, w4, w3, w2, w1, and w0 in order). In this way, the queue with the lowest
priority can get 5 Mbps (100
of SP queue-scheduling that the packets in queues with lower priority may not get service fo
is avoided. Another advantage of WRR queue is that: though the queues are scheduled in order, the
ser ice time for each queue is not fixed; that is to say, if a queue is empty, the next queue will be v
sche duled. In this way, the bandwidth resources are made full use.

Congestion Avoidance

Congestion may cause network resource unavailable and thus need to be prevented. As a type of flow
control mechanism, congestion avoidance aims to relieve network load through traffic adjusting. With
congestion avoidance configuration performed, packets are dropped in advance when the utilization of
certain network resources (such as output queues or buffer created in
degree.
Traditional pack et dropping policy
Tail drop is adopted in traditional packet drop policies. It drops all the newly arrived packets when the
current queue length reaches a specific value.
Such a policy will result in global
TCP connections simultaneously
and slow startup for the t
Consequently, the network traffic jitters all the tim
WRED
Y ou can use weighted random ea
Mbps × 1/(5+5+3+3+1+1+1+1)) bandwidth at least, and the disadvantage
TCP connection synchronization. If a queue drops packets of multiple
, the TCP connections will turn to the state of congestion avoidance
raffics to be regulated. The traffic peak will then occur in a certain future time.
rly detection (WRE
45-12
ch port. WRR configures a weight value for
e.
D) to avo id global TCP session synchronization.
r a long time
the memory) reaches certain