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INCIDENT #165 · ROUTING

Service edge: The second client subnet gets no reply

Hard25 minutesPro lab

The incident

Internal API clients and an independent observability edge share a transit network with an isolated partner segment. Every core router peers with the other three, between separate client and service handoffs. Requests reach both service networks, but the second client subnet cannot receive replies. The first client subnet still works. Investigate internal API transport from R1: 10.171.128.0/24 to 10.171.138.0/24 (TCP/8443), and 10.171.129.0/24 to 10.171.139.0/24 (TCP/443).

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

Service edge: The second client subnet gets no reply — 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 · 10.171.144.0/30R2 ↔ R3 · 10.171.145.0/30R2 ↔ R4 · 10.171.146.0/30R2 ↔ R5 · 10.171.147.0/30R3 ↔ R4 · 10.171.148.0/30R3 ↔ R5 · 10.171.149.0/30R4 ↔ R5 · 10.171.150.0/30R5 ↔ R6 · 10.171.151.0/30R1Client 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 R210.171.144.0/30
  • R2 R310.171.145.0/30
  • R2 R410.171.146.0/30
  • R2 R510.171.147.0/30
  • R3 R410.171.148.0/30
  • R3 R510.171.149.0/30
  • R4 R510.171.150.0/30
  • R5 R610.171.151.0/30
6 devices · Cisco IOS syntax · Simulated network

What you’ll practice

return pathstatic routingbidirectional verification

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