IronPort Systems ES-1528 - Manual (Page 71)

ES-1528 User’s Guide
71
C
H A P T E R
11
QoS
This chapter introduces the quality of service (QoS) parameters you can configure on the
switch.
11.1 QoS Overview
QoS is used to help solve performance degradation when there is network congestion. Use the
QoS Setting screen to configure queuing algorithms for outgoing traffic.
Queuing algorithms allow switches to maintain separate queues for packets from each
individual source or flow and prevent a source from monopolizing the bandwidth.
11.1.1 Weighted Round Robin (WRR)
Round Robin scheduling services queues on a rotating basis and is activated only when a port
has more traffic than it can handle. A queue is given an amount of bandwidth irrespective of
the incoming traffic on that port. This queue then moves to the back of the list. The next queue
is given an equal amount of bandwidth, and then moves to the end of the list; and so on,
depending on the number of queues being used. This works in a looping fashion until a queue
is empty.
Weighted Round Robin (WRR) scheduling uses the same algorithm as round robin
scheduling, but services queues based on their priority and queue weight (the number you
configure in the queue Weight field) rather than a fixed amount of bandwidth. WRR is
activated only when a port has more traffic than it can handle. Queues with larger weights get
more service than queues with smaller weights. This queuing mechanism is highly efficient in
that it divides any available bandwidth across the different traffic queues and returns to queues
that have not yet emptied.
11.1.2 Strict Priority
Strict priority scheduling singles out the highest priority queue and ensures all queued traffic
in this queue is transmitted before servicing the lower priority queues. Strict priority
scheduling services the remaining queues using WRR. As traffic comes into the switch, traffic
on the highest priority queue, Queue 3 is transmitted first. Only when that queue empties,
traffic on the lower priority queues is transmitted using WRR scheduling.