Branch WAN: A healthy control plane hides a dropped host
The incident
Branch resolvers reach a central DNS service across a regional aggregation router and a WAN handoff. Traffic crosses three routed handoffs before reaching the service gateway. OSPF neighbors and the service /24 route look healthy, but one host at .10 is unreachable. Another host in the same subnet responds. Investigate central DNS transport from R1: 10.164.160.0/24 to 10.164.168.0/24 (TCP/53), and 10.164.161.0/24 to 10.164.169.0/24 (UDP/53).
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
Link addressing (4)
- R1 R2
10.164.176.0/30 - R2 R3
10.164.177.0/30 - R3 R4
10.164.178.0/30 - R4 R5
10.164.179.0/30
What you’ll practice
Interview exercises are inspired by common industry competencies. They are not proprietary interview questions.