10.1 Queuing
10.1.1 Queuing types

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.