Explanation/Reference:
Section: Considerations for Expanding an Existing Network Explanation
Explanation:
Low-latency queuing (LLQ) provides bandwidth and delay guarantees through the creation of one or more strict-priority queues that can be used specifically for delay-sensitive traffic, such as voice and video traffic.
In addition, LLQ supports the creation of up to 64 user-defined traffic classes. Each strict-priority queue can use as much bandwidth as possible but can only use its guaranteed minimum bandwidth when other queues have traffic to send, thereby avoiding bandwidth starvation for the user-defined queues. Cisco recommends limiting the strict-priority queues to a total of 33 percent of the link capacity.
Class-based weighted fair queuing (CBWFQ) provides bandwidth guarantees, so it can be used for voice, video, and mission-critical traffic. However, CBWFQ does not provide the delay guarantees provided by LLQ, because CBWFQ does not provide support for strict-priority queues. CBWFQ improves upon weighted fair queuing (WFQ) by enabling the creation of up to 64 custom traffic classes, each with a guaranteed minimum bandwidth.
Although WFQ can be used for voice, video, and mission-critical traffic, it does not provide the bandwidth or delay guarantees provided by LLQ, because WFQ does not support the creation of strict-priority queues. Traffic flows are identified by WFQ based on source and destination IP address, port number, protocol number, and Type of Service (ToS). Although WFQ is easy to configure, it is not supported on high-speed links. WFQ is used by default on Cisco routers for serial interfaces at 2.048 Mbps or lower.
First-in-first-out (FIFO) queuing does not provide any traffic guarantees of any sort. FIFO queuing requires no configuration, because all packets are arranged into a single queue. As the name implies, the first packet received is the first packet transmitted, without regard for packet type, protocol, or priority.
Therefore, FIFO queuing is not appropriate for voice, video, or mission-critical traffic. By default, Cisco uses FIFO queuing for interfaces faster than 2.048 Mbps.
Reference:
CCDA 200-310 Official Cert Guide, Chapter 6, Low-Latency Queuing, p. 235 Cisco: Enterprise QoS Solution Reference Network Design Guide: Queuing and Dropping Principles Cisco: Signalling Overview: RSVP Support for Low Latency Queuing