When a packet arrives on a router’s interface, a protocol-independent switching process handles
it. The router then switches the traffic to the outgoing interface buffer. An example of a protocolindependent
switching process is first-in, first-out (FIFO), which is the original algorithm for
packet transmission. FIFO was the default for all routers until weighted fair queuing (WFQ) was
developed. The problem with FIFO is that transmission occurs in the same order as messages are
received. If an application such as Voice over IP (VoIP) required traffic to be reordered, the network
engineer needed to establish a queuing policy other than FIFO queuing.
Cisco IOS software offers three queuing options as an alternative to FIFO queuing:
Weighted fair queuing (WFQ) prioritizes interactive traffic over file transfers to ensure
satisfactory response time for common user applications.
Priority queuing ensures timely delivery of a specific protocol or type of traffic that is transmitted
before all others.
Custom queuing establishes bandwidth allocations for each type of traffic.
We will discuss these three queuing options in detail in the “IOS Queuing Options” section later
in this chapter.