Dual ISP BGP Route Withdrawal Causing Complete GPU Cloud Region Outage
Lambda Labs' us-south-3 region experienced a complete network outage when both redundant ISPs simultaneously stopped advertising BGP routes. The primary fiber link was damaged the prior day, shifting traffic to the secondary ISP. When the secondary ISP also stopped advertising routes, all network connectivity to the region was lost, making cloud instances inaccessible for an extended period.
Lambda Labs' us-south-3 region experienced a complete network outage when both redundant ISPs simultaneously stopped advertising BGP routes.
What this failure is
Dual ISP BGP Route Withdrawal Causing Complete GPU Cloud Region Outage is a Infrastructure failure seen during ML training runs. Lambda Labs' us-south-3 region experienced a complete network outage when both redundant ISPs simultaneously stopped advertising BGP routes. The primary fiber link was damaged the prior day, shifting traffic to the secondary ISP. When the secondary ISP also stopped advertising routes, all network connectivity to the region was lost, making cloud instances inaccessible for an extended period. Common tags: Lambda, Bgp, Isp, Network Outage.
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Why it happens (the mechanism)
Primary ISP fiber link was physically damaged the day before the outage, shifting all traffic to secondary ISP. Secondary ISP suffered a separate physical fiber cut or routing configuration issue that stopped BGP advertisement. Complete loss of route advertisement left the region's IP space unreachable from any upstream. Taken together, these mechanisms explain why the failure is reproducible, why it tends to surface on specific workloads or scales, and why generic mitigation attempts often fall short without addressing the underlying cause.
What you'll observe
- All cloud instances in a region become unreachable from the public internet
- Both redundant network providers have simultaneously lost connectivity
- No BGP routes advertised for the region's IP space
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| Users cannot SSH or connect to instances in the affected region | Primary ISP fiber link was physically damaged the day before the outage, shifting all traffic to secondary ISP |
| BGP looking glass shows no routes advertised for the region prefix | Secondary ISP suffered a separate physical fiber cut or routing configuration issue that stopped BGP advertisement |
| Network connectivity to the data center is completely severed | Complete loss of route advertisement left the region's IP space unreachable from any upstream |
Which systems are affected
- GPU cloud regions relying on 2 or fewer ISPs for network transit
- Regions without redundant fiber path diversity (different physical routes)
- Infrastructure without BGP session monitoring and alerting
How to confirm this is the problem
Use this checklist to test the hypothesis against a small reproduction. No single line proves the root cause, so preserve the preceding events and compare one variable at a time.
- ✓Reproduce the failure from a clean checkpoint/seed: the symptom must appear without warm-up state from a previous run.
- ✓Verified signal present: Users cannot SSH or connect to instances in the affected region
- ✓Verified signal present: BGP looking glass shows no routes advertised for the region prefix
- ✓Verified signal present: Network connectivity to the data center is completely severed
- ✓A targeted fix from the "How to fix it" section eliminates or substantially reduces the symptom within one validation pass.
The fix and the prevention pattern
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Root cause
- Primary ISP fiber link was physically damaged the day before the outage, shifting all traffic to secondary ISP
- Secondary ISP suffered a separate physical fiber cut or routing configuration issue that stopped BGP advertisement
- Complete loss of route advertisement left the region's IP space unreachable from any upstream
The fix and how to prevent it
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