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INCIDENT #027 · OSPF

The backbone boundary

Medium25 minutesPro lab

The incident

Newark data center. Two small routed rings share a regional interchange; the service sits beyond the second ring. The directly connected service-edge link responds, but the OSPF adjacency across it is absent. Other adjacencies and monitoring traffic remain healthy. Investigate the two service networks from R1: 10.204.96.0/24 to 10.204.106.0/24 (TCP/443), and 10.204.97.0/24 to 10.204.107.0/24 (TCP/8443).

Your objective

Restore both authorized service transports and return reachability, including clients and service hosts in both halves of the primary subnet. Keep R2 blocked from both service transports, keep SSH blocked to both service subnets, and preserve monitoring between R1 and R4. Do not change physical topology or addressing. These are network forwarding checks, not application-level transactions.

The network

Network topology6 devices 7 links

The backbone boundary — network topologyR1: Client metro. R2: Partner metro. R3: Interchange. R4: North service metro. R5: South service metro. R6: Service gateway. R1 connects to R2. R2 connects to R3. R3 connects to R1. R3 connects to R4. R4 connects to R5. R5 connects to R3. R5 connects to R6. Connections show physical links, not service health.R1 ↔ R2 · 10.204.112.0/30R2 ↔ R3 · 10.204.113.0/30R3 ↔ R1 · 10.204.114.0/30R3 ↔ R4 · 10.204.115.0/30R4 ↔ R5 · 10.204.116.0/30R5 ↔ R3 · 10.204.117.0/30R5 ↔ R6 · 10.204.118.0/30R1Client metroR1 · Client metro · Starting deviceR2Partner metroR2 · Partner metroR3InterchangeR3 · InterchangeR4North servicemetroR4 · North service metroR5South servicemetroR5 · South service metroR6ServicegatewayR6 · Service gateway
Physical links · Service health not shown
Link addressing (7)
  • R1 R210.204.112.0/30
  • R2 R310.204.113.0/30
  • R3 R110.204.114.0/30
  • R3 R410.204.115.0/30
  • R4 R510.204.116.0/30
  • R5 R310.204.117.0/30
  • R5 R610.204.118.0/30
6 devices · Cisco IOS syntax · Simulated network

What you’ll practice

ospf adjacencyarea membershipconnected vs learned

Interview exercises are inspired by common industry competencies. They are not proprietary interview questions.