Fault Tolerance & Redundancy

You’ll be able to

Define fault tolerance and explain why the Internet needs it
Explain how redundancy provides alternate paths and copies
Relate redundant routing to the Internet’s reliability

What fault tolerance means

Fault tolerance is a system’s ability to keep working correctly even when some of its parts fail. In a system as vast as the Internet, failures are constant — cables get cut, routers crash, power goes out somewhere every day. A fault-tolerant design ensures that a single failure does not bring the whole system down. This is not achieved by making every part perfect (impossible at scale) but by building the system so it can route around or absorb failures.

Redundancy: the key mechanism

The main way to achieve fault tolerance is redundancy — deliberately having more than one of something so that a backup exists when one part fails. On the Internet, redundancy takes the form of multiple physical connections between networks, so there are several possible paths between any two points. If one path goes down, packets are simply routed along another. Data centers also keep redundant copies of data and redundant power supplies. Redundancy costs more resources, but it buys reliability.

Why the Internet stays up

The Internet’s reliability comes directly from combining packet routing with redundant paths. Because packets can travel any of many routes and routers continuously choose working paths, the failure of a single router or cable just diverts traffic elsewhere — often without users noticing. A network with only one path between two points has a single point of failure: if that link breaks, communication stops entirely. Redundancy removes single points of failure, which is exactly what makes the Internet fault tolerant.

Fault tolerance through redundancy

more redundant paths/copies → fewer single points of failure → higher fault tolerance

Redundancy means having backups (extra connections, extra copies) so that when one part fails, the system continues using another.

Worked example

Network A connects two cities with a single cable. Network B connects the same cities with three separate cables along different routes. A backhoe cuts one cable in each network. Compare what happens.

1.Network A had only one path — a single point of failure. Cutting its one cable severs the connection entirely; communication stops.
2.Network B has redundancy: three independent paths. Cutting one still leaves two working routes.
3.In Network B, routers detect the broken link and send packets along the remaining cables, so communication continues.
4.Network B is fault tolerant because redundancy provided alternate paths; Network A was not.

Answer: Network A fails completely — its single cable is a single point of failure. Network B keeps working because its redundant cables give alternate paths for packets to route around the cut. Redundancy is what makes Network B fault tolerant.

Checkpoint

What is the primary benefit of building redundancy (multiple connections and copies) into a network?

Watch out

A single point of failure is any part whose failure stops the whole system. Redundancy exists specifically to eliminate these. A network with just one path between two points is not fault tolerant.

Checkpoint

A company’s network has exactly one router connecting its office to the Internet, with no backup. What is the main weakness of this design?

On the exam

Link the concepts in a chain: redundancy → multiple paths → no single point of failure → fault tolerance. If an exam scenario has only one path or one copy, its weakness is the single point of failure.

Answer the 2 checkpoints as you read.