Branch WAN: One subnet is absent from the routing domain
The incident
Branch resolvers reach a central DNS service across a regional aggregation router and a WAN handoff. Two triangular regional cores meet at a WAN handoff, with the service gateway beyond the remote core. OSPF neighbors are established and the primary service works. Only the secondary service prefix is missing from remote route tables. Investigate central DNS transport from R1: 10.164.64.0/24 to 10.164.76.0/24 (TCP/53), and 10.164.65.0/24 to 10.164.77.0/24 (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 R5. Do not change physical topology or addressing. These are network forwarding checks, not application-level transactions.
The network
Link addressing (8)
- R1 R2
10.164.80.0/30 - R2 R3
10.164.81.0/30 - R3 R1
10.164.82.0/30 - R3 R4
10.164.83.0/30 - R4 R5
10.164.84.0/30 - R5 R6
10.164.85.0/30 - R6 R4
10.164.86.0/30 - R6 R7
10.164.87.0/30
What you’ll practice
Interview exercises are inspired by common industry competencies. They are not proprietary interview questions.