How does redundancy make a network fault tolerant, so it keeps working when parts fail?
Topic 4.2 Fault Tolerance: a system is fault tolerant if it continues to operate when some components fail; redundancy (multiple paths or copies) is the main way networks achieve fault tolerance.
A focused answer to AP CSP Topic 4.2, covering what fault tolerance means, how redundancy of paths and data provides it, why the Internet is fault tolerant, the difference between a fault-tolerant and a non-redundant system, and the costs of redundancy.
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What this topic is asking
The College Board (Topic 4.2) wants you to understand fault tolerance: a system's ability to keep operating when some of its components fail. The main way networks achieve fault tolerance is redundancy: providing multiple paths or duplicate copies so there is no single point of failure. You need to explain why the Internet is fault tolerant, contrast a redundant design with a non-redundant one, and recognize that redundancy has costs as well as benefits.
What fault tolerance means
Redundancy: the main technique
Single point of failure
A single point of failure is any component whose failure stops the entire system because everything depends on it and there is no backup. A network with only one cable between two halves has a single point of failure: cut the cable and the halves cannot communicate. Redundancy removes single points of failure by providing alternatives.
Why the Internet is fault tolerant
The Internet was designed with redundant routing: there are typically many possible paths between any two devices. When a connection or router fails, packets are rerouted along a different path, so communication continues. This is why a single outage rarely takes the whole Internet down.
Try this
Q1. Define fault tolerance. [1 point]
- Cue. The ability of a system to continue operating correctly even when some of its components fail.
Q2. State one cost of building redundancy into a network. [1 point]
- Cue. It requires extra hardware (more connections or duplicate components) and adds complexity and expense.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2022 (style)1 marksMultiple choice. A computer network is designed so that there are several different routes between any two computers. What property does this design give the network?
(A) Lower bandwidth on every connection.
(B) Fault tolerance, because data can be rerouted if one path fails.
(C) A guarantee that no data is ever lost.
(D) A single point of failure.
Show worked answer →
The answer is (B).
Multiple routes between points provide redundancy, which gives fault tolerance: if one path fails, data is rerouted along another, so the network keeps working. (A) is wrong: redundancy does not reduce bandwidth. (C) is too strong: fault tolerance improves reliability but does not guarantee zero loss. (D) is the opposite: redundancy removes single points of failure.
Markers reward linking multiple paths (redundancy) to fault tolerance through rerouting.
AP 2021 (style)2 marksFree response (short). Explain what it means for a system to have a single point of failure, and how redundancy removes it.
Show worked answer →
A 2-point question on single points of failure and redundancy.
Point 1: A single point of failure is a component whose failure causes the whole system to stop working, because everything depends on it and there is no alternative.
Point 2: Redundancy adds duplicate components or alternative paths, so if one fails, another takes over and the system keeps running. This removes the single point of failure and makes the system fault tolerant. A common error is to think redundancy just adds cost with no benefit; its benefit is continued operation despite failures.
Related dot points
- Topic 4.1 The Internet: the Internet is a network of networks that moves data in packets using protocols such as IP and TCP, with addressing, routing and standards enabling scalable communication.
A focused answer to AP CSP Topic 4.1, covering the Internet as a network of networks, IP addresses, packets and packet switching, protocols (IP, TCP, HTTP, DNS), bandwidth and latency, redundancy in routing, and why open standards enable scalability.
- Topic 4.3 Parallel and Distributed Computing: parallel computing runs tasks simultaneously on multiple processors and distributed computing spreads work across multiple computers, improving speed but with limits.
A focused answer to AP CSP Topic 4.3, covering sequential versus parallel computing, distributed computing, speedup and its calculation, why some tasks cannot be fully parallelised, the benefits of solving large problems, and worked speedup reasoning.
- Topic 5.6 Safe Computing: personal data is collected and stored by computing systems, and safe computing uses authentication, encryption and awareness of threats such as malware and phishing to protect it.
A focused answer to AP CSP Topic 5.6, covering how personal data is collected and tracked, privacy risks, authentication and strong passwords, multi-factor authentication, encryption (symmetric and public key), and common threats such as malware and phishing, with practical safeguards.
- Topic 2.4 Using Programs with Data: programs process large data sets through cleaning, filtering, classifying and transforming data, often using lists and iteration to scale to large amounts of data.
A focused answer to AP CSP Topic 2.4, covering why programs are essential for large data sets, cleaning and classifying data, filtering with conditionals, using lists and iteration to process data at scale, and visualizing results, with worked pseudocode.
- Topic 5.1 Beneficial and Harmful Effects: computing innovations have both beneficial and harmful effects on society, economy and culture, and effects may be intended or unintended.
A focused answer to AP CSP Topic 5.1, covering how a single computing innovation can have both beneficial and harmful effects, intended versus unintended consequences, effects on individuals and society, and how to analyze an innovation's impact for the exam.
Sources & how we know this
- AP Computer Science Principles Course and Exam Description — College Board (2025)