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INCIDENT #187 · OSPF

Backup network: One subnet is absent from the routing domain

Easy15 minutesPro lab

The incident

Backup agents traverse a routed storage boundary while replication monitoring and a lab tenant share a separate branch. Parallel three-site metro paths have cross-links at each stage, between independent client and service handoffs. OSPF neighbors are established and the primary service works. Only the secondary service prefix is missing from remote route tables. Investigate backup transport from R1: 10.174.64.0/24 to 10.174.78.0/24 (TCP/2049), and 10.174.65.0/24 to 10.174.79.0/24 (UDP/111).

Your objective

Restore both authorized service transports and return reachability, including clients and service hosts in both halves of the primary subnet. Keep R4 blocked from both service transports, keep SSH blocked to both service subnets, and preserve monitoring between R1 and R5. Do not change physical topology or addressing. These are network forwarding checks, not application-level transactions.

The network

Network topology8 devices 9 links

Backup network: One subnet is absent from the routing domain — network topologyR1: Client edge. R2: West north. R3: Central north. R4: East north. R5: West south. R6: Central south. R7: East south. R8: Service gateway. R1 connects to R2. R2 connects to R3. R3 connects to R4. R5 connects to R6. R6 connects to R7. R2 connects to R5. R3 connects to R6. R4 connects to R7. R7 connects to R8. Connections show physical links, not service health.R1 ↔ R2 · 10.174.80.0/30R2 ↔ R3 · 10.174.81.0/30R3 ↔ R4 · 10.174.82.0/30R5 ↔ R6 · 10.174.83.0/30R6 ↔ R7 · 10.174.84.0/30R2 ↔ R5 · 10.174.85.0/30R3 ↔ R6 · 10.174.86.0/30R4 ↔ R7 · 10.174.87.0/30R7 ↔ R8 · 10.174.88.0/30R1Client edgeR1 · Client edge · Starting deviceR2West northR2 · West northR3Central northR3 · Central northR4East northR4 · East northR5West southR5 · West southR6Central southR6 · Central southR7East southR7 · East southR8ServicegatewayR8 · Service gateway
Physical links · Service health not shown
Link addressing (9)
  • R1 R210.174.80.0/30
  • R2 R310.174.81.0/30
  • R3 R410.174.82.0/30
  • R5 R610.174.83.0/30
  • R6 R710.174.84.0/30
  • R2 R510.174.85.0/30
  • R3 R610.174.86.0/30
  • R4 R710.174.87.0/30
  • R7 R810.174.88.0/30
8 devices · Cisco IOS syntax · Simulated network

What you’ll practice

ospf advertisementconnected networksroute lookup

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