Explanation/Reference:
Section: Considerations for Expanding an Existing Network Explanation
Explanation:
Of the choices provided, low-latency queuing (LLQ) is the most appropriate queuing method for handling voice, video, mission-critical, and lower-priority traffic. LLQ supports the creation of up to 64 user-defined traffic classes as well as one or more strict-priority queues that can be used to guarantee bandwidth for delay-sensitive traffic, such as voice and video traffic. Each strict-priority queue can use as much bandwidth as possible but can use only the guaranteed bandwidth when other queues have traffic to send, thereby avoiding bandwidth starvation. 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, mission-critical, and lower-priority 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, mission-critical, and lower-priority traffic, it does not provide the bandwidth guarantees or the strict-priority queues that are provided by LLQ. WFQ is used by default on Cisco routers for serial interfaces at 2.048 Mbps or lower. 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.
First-in-first-out (FIFO) queuing is the least appropriate for voice, video, mission-critical, and lower-priority traffic. By default, Cisco uses FIFO queuing for interfaces faster than 2.048 Mbps. 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.
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 Queueing