FIFO is by far the simplest queuing technique, and is the default method used on Cisco router interfaces that have more than 2.048 MB of bandwidth available. When an interface is using FIFO, packets are added to a single queue, and are processed in the order the router receives them. FIFO provides a few key benefits. First are foremost, it is not very computationally taxing on the router, and the traffic flow couldn’t possibly be more predictable. This makes it a reasonable choice for interfaces that have a great deal of bandwidth available, and a relatively light load.
On the downside, FIFO does nothing to recognize that traffic from one application may be more time-sensitive than from another. For example, a network that relies on time-critical SNA traffic might experience delays at the expense of large FTP data transfers, since the router will simply process packets in the order they are received. This can also be a problem for applications like VoIP, which rely on data reaching its destination in a timely manner. FIFO queuing on congested networks can lead to latency, jitter, and even packet loss.