Which of the following terms describes LFI?
- a QoS model
- a queuing method
- a link efficiency mechanism
- a resource reservation method
- a congestion avoidance mechanism
Link fragmentation and interleaving (LFI) is a link efficiency mechanism. Without LFI, small packets, such as voice packets, can become stuck behind large packets as the large packets are transmitted through an interface, thereby causing delay and jitter. LFI breaks up large data packets into small pieces and then interleaves the small packets among the fragments of the large packet. Another link efficiency mechanism used by Voice over IP (VoIP) is Compressed Real-time Transport Protocol (cRTP), which compresses the Layer 3 and Layer 4 headers of voice packets to a fraction of their original size.
LFI is a Quality of Service (QoS) feature, not a QoS model. QoS enables a network to treat a specific type of traffic with a different priority than other types of traffic. For example, QoS can ensure that voice traffic gets higher priority on a network than data traffic. QoS models include the best-effort model, the Integrated Services (IntServ) model, and the Differentiated Services (DiffServ) model. Each QoS model handles packet flows in a different manner. For example, IntServ requires that applications reserve their end-to-end bandwidth requirements, and DiffServ prioritizes packets by traffic class. Because of some inherent shortcomings in the IntServ model, Cisco recommends using DiffServ when delivering voice traffic.
LFI is not a resource reservation method. When IntServ is used, bandwidth resources must be reserved for each traffic flow. These resources are reserved from the source to the destination by Resource Reservation Protocol (RSVP).
LFI is not a queuing method. Queuing methods deal with congestion management. Each queuing method handles network congestion in a different manner. Queuing methods employed by Cisco routers include first-in-first-out (FIFO) queuing, priority queuing (PQ), custom queuing (CQ), round robin (RR), weighted RR (WRR), weighted fair queuing (WFQ), class-based WFQ (CBWFQ), and low latency queuing (LLQ). By default, Cisco devices use FIFO queuing for interfaces faster than 2.048 Mbps and use WFQ for serial interfaces at 2.048 Mbps or lower.
LFI is not a congestion avoidance mechanism. Congestion avoidance mechanisms mitigate tail drop, which occurs when a router drops new packets because its queues are too full to accept them. Congestion avoidance mechanisms employed by Cisco routers include random early detection (RED), weighted RED (WRED), and class-based WRED (CBWRED).