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March 2025

18

Watch our 'Azure Incident Retrospective' video about this incident:

What happened?

Between 13:37 and 16:52 UTC on 18 March, and again between 23:20 UTC on 18 March and 00:30 UTC on 19 March, 2025, a combination of a third-party fiber cut, and an internal tooling failure resulted in an impact to a subset of Azure customers with services in our East US region.

During the first impact window, immediately after the fiber cut, customers may have experienced intermittent connectivity loss for inter-zone traffic that included AZ03 - to/from other zones, or to/from the public internet. During this time, the traffic loss rate peaked at 0.02% for short periods of time. Traffic within AZ03, as well as traffic to/from/within AZ01 and AZ02, was not impacted.

During the second impact window, triggered by the tooling issue, customers may have experienced intermittent connectivity loss – primarily when sending inter-zone traffic that included AZ03. During this time, the traffic loss rate peaked at 0.55% for short periods of time. Traffic entering or leaving the East US region was not impacted, but there was some minimal impact to inter-zone traffic from both of the other Availability Zones, AZ01 and AZ02.

Note that the 'logical' availability zones used by each customer subscription may correspond to different physical availability zones. Customers can use the Locations API to understand this mapping, to confirm which resources run in this physical AZ, see: .

What went wrong and why?

At 13:37 UTC on 18 March 2025, a drilling operation near one of our network paths accidentally struck fiber used by Microsoft, causing an unplanned disruption to datacenter connectivity within AZ03. When fiber cuts impact our networking capacity, our systems are designed to redistribute traffic automatically to other paths. In this instance, we had two concurrent failures happen – before the cut, a datacenter router in AZ03 was down for maintenance and was in the process of being repaired. This combination of multiple concurrent failures impacted a small portion of our diverse capacity within AZ03, leading to the potential for intermittent connectivity issues for some customers. At 13:55 UTC on 18 March 2025, we began mitigating the impact of the fiber cut by load balancing traffic and restoring some of the impacted capacity – customers would have started to see their services recover at this time.

Additionally, after the fiber cut and failed isolation, at 14:16 UTC a linecard failed on another router further reducing overall capacity to AZ03. However, as traffic had been re-routed, this further reduction in capacity did not cause any additional customer impact.

During the initial mitigation efforts outlined above, our auto-mitigation tool encountered a lock contention problem blocking commands on the impacted devices, failing to isolate all capacity connected to those devices. This failure left some of the impacted capacity un-isolated, and our system did not flag this failed isolation state. Due to some capacity being out of service from the fiber cut, this failed state was not immediately flagged in our systems as the down capacity was not carrying production traffic.

At approximately 21:00 UTC, our fiber provider commenced recovery work on the damaged fiber. During the capacity recovery process, at 23:20 UTC, as a result of the failure to isolate all the impacted fiber capacity, as individual fibers were repaired, our recovery systems begin re-sending traffic to the devices connected to the un-isolated capacity, therefore, bringing them back into service without safe levels of capacity. This caused traffic congestion that impacted customers as described above.

The traffic congestion within AZ03, due to the tooling failure, triggered an unplanned failure mode on a regional hub router that connects multiple datacenters. By design, our network devices attempt to contain congestive packet loss to capacity that is already impacted. Due to the encountered failure mode, this containment failed on a subset of routers – so congestion spread to neighboring capacity on the same regional hub router, beyond AZ03. This containment failure impacted a small subset of traffic from the regional hub router to AZ1 and AZ2.

At this stage, all originally-impacted capacity from the third-party fiber cut was manually isolated from the network – mitigating all customer impact by 00:30 UTC on 19 March. At 01:52 UTC on 19 March the underlying fiber cut was fully recovered. At that time, we completed the test and restoration of all capacity to pre-incident levels by 06:50 UTC on 19 March.

How did we respond?

  • 13:37 UTC on 18 March 2025 – Customer impact began, triggered by a fiber cut causing network congestion which led to customers experiencing packet drops or intermittent connectivity. Our monitoring systems identified the impact immediately, so our on-call engineers engaged to investigate.
  • 13:45 UTC on 18 March 2025 – Our fiber provider was notified of the fiber cut and prepared for dispatch.
  • 13:55 UTC on 18 March 2025 – Mitigation efforts began identifying the impacted datacenters and redirecting traffic to healthier routes.
  • 15:07 UTC on 18 March 2025 – All customers using the East US region were notified about connectivity issues, even if their services were not directly impacted.
  • 16:52 UTC on 18 March 2025 – Mitigation efforts were successfully completed. All devices affected by the fiber cut were isolated, all customer traffic was using healthy paths and not experiencing congestion.
  • 23:20 UTC on 18 March 2025 – Customer impact recommenced, due to a tooling failure during the capacity repair process of the initial fiber cut.
  • 00:30 UTC on 19 March 2025 – This impact was mitigated after isolating the capacity that was incorrectly added by the tooling failure as part of the recovery process. Customers and services would have experienced full mitigation.
  • 01:52 UTC on 19 March 2025 – The underlying fiber cut was fully restored. We continued to monitor our capacity during the recovery process.
  • 06:50 UTC on 19 March 2025 – Fiber restoration efforts were completed. The incident was confirmed as mitigated.

How are we making incidents like this less likely or less impactful?

  • We are fixing the tooling failure that caused the devices to be restored to take traffic before they were production ready. (Estimated completion: May 2025)
  • We are expediting a capacity upgrade within the most impacted datacenter, ahead of a planned technology refresh for all datacenters within this region - to de-risk the impact of multiple concurrent failures. (Estimated completion: July 2025)
  • In the longer term, we are working to limit the scope of impact further – specifically, to prevent the failure of a device from spreading across availability zones. (Estimated completion: February 2026) 

How can customers make incidents like this less impactful?

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