| A network's responsiveness is
typically measured by its end users as they access the network to
perform day-to-day tasks. Today's users expect network resources to
respond quickly, as if network applications were running from a
local hard drive. You must tailor networks to meet the needs of
applications, especially delay sensitive applications such as voice and video. The Cisco IOS offers traffic
prioritization features to tune responsiveness in a congested
network. Routers can be configured to prioritize certain kinds of
traffic based on protocol information, such as TCP port numbers. As
shown in the figure, traffic prioritization ensures that packets
carrying mission-critical data take precedence over less important
traffic.
If the router schedules these packets
for transmission on a first-come, first-served basis, users could
experience an unacceptable lack of responsiveness. Therefore, an end
user sending delay-sensitive voice traffic, may be forced to wait
too long while the router empties its buffer of a long train of
queued packets.
The Cisco IOS addresses priority and
responsiveness issues through queuing. The question of priority is
most important on routers that maintain a slow WAN connection and
therefore experience frequent congestion. Queuing refers to the process
that the router uses to schedule packets for transmission during
periods of congestion. By using the queuing feature, you can
configure a congested router to reorder packets so that
mission-critical and delay-sensitive traffic is sent out first.
These higher priority packets are sent first even if other
low-priority packets arrive first. The Cisco IOS supports four
methods of queuing, as described in the following sections:
first-in, first out (FIFO) queuing; priority queuing; custom
queuing, and weighted fair queuing (WFQ). Only one of these queuing
methods can be applied per interface because each method handles
traffic in a unique way.
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