OSPFv2 Multi-Area with ABR Summarization (Redundant Core)

Bundle lab

Deploy a 5-node OSPFv2 lab featuring a redundant area 0 triangle (R1–R2–R3) and an ABR (R3) connecting to area 1 with branch networks on R4. A client on R1’s area 0 LAN validates reachability to branch loopback networks summarized by the ABR. You will set explicit router-ids from Loopback0, use passive-interface default, advertise R4’s loopbacks as /24s using ip ospf network point-to-point, and summarize area 1 into a /23 on the ABR. Verify FULL adjacencies, a single O IA summary on R1, and end-to-end connectivity. Troubleshoot an introduced area mismatch and interface/addressing issues.

Learning objectives

• Build an OSPFv2 multi-area design with a redundant area 0 triangle and an ABR to area 1
• Set and advertise explicit OSPF router-ids from Loopback0
• Use passive-interface default and selectively enable OSPF on transit links only
• Advertise /24 loopbacks using ip ospf network point-to-point semantics
• Summarize area 1 networks with area range and validate a single O IA summary in area 0
• Verify adjacencies, LSA types, and route tables; test redundancy and failover
• Troubleshoot area mismatches, neighbor states, and route propagation issues

Troubleshooting focus

• A backbone adjacency is stuck in EXSTART/LOADING or never forms: Compare area assignments on both ends of the link. Core links must be area 0 on both ends; the R3–R4 link must be area 1.; Run show ip interface brief to confirm the interfaces are up/up and the subnets on each side match — no /30 overlaps or typos.; If the neighbor never appears at all, confirm you un-passived the correct transit interface(s).
• R3 sees R4's loopbacks as /32s: the point-to-point network type was not applied on Loopback1 and Loopback2. Fix it, then clear the OSPF process or wait for the refresh.
• show ip route ospf on R1 shows two /24s instead of a single 10.1.20.0/23: confirm the ABR's area range command and that 10.1.20.0/24 and 10.1.21.0/24 are truly contiguous.
• Pings from CLIENT01 fail: trace the first failing hop with traceroute, then inspect that node's route table and its OSPF neighbor and LSDB state.