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INCIDENT #123 · NETWORK OPERATIONS

Data center: One service address drops out

Medium25 minutesPro lab

The incident

Application workers connect to database and connection-pool subnets through separate access and service aggregation layers. Every core router peers with the other three, between separate client and service handoffs. The service at .10 is unreachable, but .200 in the same subnet and the secondary service still work. Investigate database connectivity from R1: 10.166.64.0/24 to 10.166.74.0/24 (TCP/5432), and 10.166.65.0/24 to 10.166.75.0/24 (TCP/6432).

Your objective

Restore both authorized service transports and return reachability, including clients and service hosts in both halves of the primary subnet. Keep R3 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 8 links

Data center: One service address drops out — network topologyR1: Client edge. R2: Ingress core. R3: South core. R4: North core. R5: Egress core. R6: Service gateway. R1 connects to R2. R2 connects to R3. R2 connects to R4. R2 connects to R5. R3 connects to R4. R3 connects to R5. R4 connects to R5. R5 connects to R6. Connections show physical links, not service health.R1 ↔ R2 · 10.166.80.0/30R2 ↔ R3 · 10.166.81.0/30R2 ↔ R4 · 10.166.82.0/30R2 ↔ R5 · 10.166.83.0/30R3 ↔ R4 · 10.166.84.0/30R3 ↔ R5 · 10.166.85.0/30R4 ↔ R5 · 10.166.86.0/30R5 ↔ R6 · 10.166.87.0/30R1Client edgeR1 · Client edge · Starting deviceR2Ingress coreR2 · Ingress coreR3South coreR3 · South coreR4North coreR4 · North coreR5Egress coreR5 · Egress coreR6ServicegatewayR6 · Service gateway
Physical links · Service health not shown
Link addressing (8)
  • R1 R210.166.80.0/30
  • R2 R310.166.81.0/30
  • R2 R410.166.82.0/30
  • R2 R510.166.83.0/30
  • R3 R410.166.84.0/30
  • R3 R510.166.85.0/30
  • R4 R510.166.86.0/30
  • R5 R610.166.87.0/30
6 devices · Cisco IOS syntax · Simulated network

What you’ll practice

longest prefix matchhost routesblackholes

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