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Routers have a
protocol-dependent switching process that handles traffic as it
arrives on an interface. The switching process includes the delivery of
traffic to an outgoing interface buffer. First-in, first-out (FIFO)
queuing is the classic algorithm for packet transmission. With FIFO,
transmission occurs in the same order as messages are received.
Until recently, FIFO queuing was the default for all router
interfaces.
The biggest problem with FIFO queuing is that
it sends out packets in the order they are received. Therefore, a
small Telnet or voice packet could get stuck behind two or more
large file transfer packets simply because of the order in which
they arrive at the router. The end result would be a delay in
service or eventual timeout of the smaller packets.
If traffic needs to be reordered, the
department or company must establish a queuing policy that is
different from FIFO queuing and ensures that sensitive traffic goes
out first (see the Figure).
Cisco IOS software offers three queuing
options as alternatives to FIFO queuing:
- Weighted Fair Queuing (WFQ)
prioritizes time sensitive interactive traffic, such as video
and voice, over file transfers to ensure satisfactory response
time for common user applications.
- Priority Queuing
(PQ) ensures the timely delivery of a specific
protocol or type of traffic because that traffic is transmitted
before all others.
- Custom Queuing
(CQ) establishes bandwidth allocations for each
different type of traffic.
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