When a subsea cable fails, internet traffic does not pass through one central switch. Network operators use protection paths and other available links—such as a second submarine cable or terrestrial fiber—to keep traffic moving where possible. Rerouting can preserve access, but it cannot guarantee normal speed: alternate routes may be longer, have limited spare capacity, or become congested.
What happens when an undersea cable breaks?
A submarine cable fault interrupts transmission on some part of a network, not necessarily every connection that depends on it. Internet traffic crosses interconnected networks run by different operators, so the impact depends on which routes are affected and what alternatives those networks can use.
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Submarine systems monitor transmission and use supervisory information to detect and localize faults. The details depend on the cable system and its equipment; ITU-T guidance describes fault detection and localization, including tests from terminal stations. The diagnosis helps repair teams identify and recover the damaged section.
Traffic continuity and physical repair are separate, related tasks: operators work to carry traffic over surviving paths while cable stakeholders and repair teams arrange work on the fault itself.
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How operators reroute traffic
1. Detect the fault and identify affected paths
Monitoring can reveal a loss of transmission and help narrow down where it occurred. Operators and cable-system organizations then assess which services and routes are affected. Public information generally does not reveal every network’s detection policy or topology, so there is no universal detection time or single response procedure.
2. Move traffic onto a protection path or another route
ITU-T guidance says traffic is usually rerouted onto a protection path when a fault in the wet plant causes loss of transmission. Depending on the network, an alternate route may use another submarine cable, terrestrial fiber across a border or region, or another available link. The Internet Society describes the general principle: traffic can be rerouted when one route stops working, but the alternate paths must exist and be usable.
This is not one automatic switch controlled by a central internet authority. Routing protocols, operator policies, cable and network ownership, available agreements, and coordination all shape which paths can carry traffic. The UK Parliament’s 2025 inquiry describes a multi-actor system with automated protocols, uneven visibility, and possible manual intervention.
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3. Check capacity and manage the load
A route that is technically available may not have enough spare capacity for all the traffic shifted onto it. Operators may adjust traffic engineering or activate additional capacity, but the usable amount depends on the route and incident; design capacity is not the same as spare capacity that can be used immediately.
4. Keep users informed while repairs are arranged
Operators, government authorities, cable-system organizations, and repair-vessel teams may each have a role. Service can remain degraded while the damaged cable is unavailable. Repair timing depends on factors such as fault location, permits, vessel movements, and sea and weather conditions.
What rerouting can—and cannot—preserve
Rerouting may keep a connection available without making it feel normal. A backup route can travel farther, increasing latency; it may also have less capacity than the failed path. If many operators shift traffic onto the same surviving links, congestion can slow service or cause intermittent access. Some users may remain connected while others experience severe degradation or outages, depending on their provider and route.
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Having multiple cables on a map does not always mean having independent backups. Routes can share landing points, corridors, or other infrastructure, creating a common point of failure. Resilience depends on actual route diversity as well as capacity and the ability to reach and use an alternate path.
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Examples from real cable disruptions
Papua New Guinea: a specific route and capacity increase
In an October 6, 2026 update, PNG DataCo said traffic from the unavailable PPC-1 Madang–Sydney path was being carried through Port Moresby over KSCN and the Coral Sea Cable System. The operator reported that activating a second 100G wavelength on the Port Moresby–Madang KSCN System 2 Express Link raised available capacity on that link from 100G to 200G to support regional and rerouted traffic. DataCo also warned that customers could still experience higher latency, degraded performance, or intermittent access on alternative paths. These figures describe that incident, not a general capacity target.
West Africa: terrestrial links helped reach another cable
The Internet Society’s 2024 West Africa Submarine Cable Outage Report says the disruption affected 13 countries, with degraded service or near-total internet outages. Local operators used cross-border terrestrial fiber to route traffic to the Equiano cable, which the report says experienced a fourfold increase in traffic. The report also describes route convergence that left seemingly separate cables exposed to a shared point of failure.
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East Africa: surviving cables can become congested
The Internet Society’s 2024 East Africa report says other working submarine cables quickly congested as they carried rerouted traffic. Liquid Intelligent Technologies and SEACOM worked together to provide alternate connectivity; Safaricom and Airtel reported activating alternative connectivity. The episode shows why rerouting can restore some connectivity without restoring normal performance.
Taiwan: rerouting alongside other resilience measures
Taiwan’s Ministry of Digital Affairs said it verified that operators completed rerouting after earthquake-related cable disruptions spanning December 2025 to January 2026. The ministry also identified satellite sites, expanded microwave capacity, and more cables as resilience measures. This illustrates one government’s oversight and coordination; it does not mean every cable outage uses satellite or microwave links.
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After the March 14, 2024 cable disruptions, Bayobab said it recovered more than 3 Tbps of capacity across its footprint by activating new cables and alternative routes. That is the company’s own reported figure, not a universal measure of how much capacity operators can restore.
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What makes a backup route useful?
There is no universally best backup. For a particular network, operators need to consider whether an alternate route is genuinely independent, reachable, available during the event, and capable of carrying the shifted load.
- Route independence: Does the path avoid the same vulnerable landing point, corridor, or shared infrastructure?
- Usable capacity: Is there spare capacity for the rerouted traffic, rather than only theoretical design capacity?
- Latency and performance: Will the alternate geography add path length or otherwise change service quality?
- Reach and topology: Can traffic get to the alternate cable through terrestrial links or another network partner?
- Operational availability: Is the route technically and contractually accessible during an incident, and can the organizations coordinate its use?
Public reports rarely expose each operator’s routing policy, exact topology, agreements, or live spare-capacity figures. Therefore, a route’s presence alone does not establish how much traffic it can carry or how quickly it can be brought into use.
Quick Recap
Sources
- ITU-T Recommendation G Suppl. 41 (07/2024), Design guidelines for optical fibre submarine cable systems
- PNG DataCo, Network outages and restoration update, October 6, 2026
- Taiwan Ministry of Digital Affairs, update created December 29, 2025 and updated January 12, 2026
- Internet Society, 2024 West Africa Submarine Cable Outage Report
- MTN Group / Bayobab, Statement Update | International Subsea Cable Break, March 19, 2024
- Internet Society, 2024 East Africa Submarine Cable Outage Report
- UK Parliament, Joint Committee on the National Security Strategy, Subsea telecommunications cables: resilience and crisis preparedness, 2025
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