OSPF multi-area is a cornerstone of scalable enterprise routing. On the CCNA 200-301 exam, you must understand how to configure and verify multi-area OSPF (exam objective 3.4). In real networks, multi-area OSPF reduces CPU and memory load, limits route propagation, and provides faster convergence. This lab walks you through a three-router multi-area topology, from configuration to verification.
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Imagine a city transportation department managing roads across three districts: Downtown (Area 0), Suburb East (Area 1), and Suburb West (Area 2). The central office (Area 0) has a master map of all roads in the city. Each district office (Area 1 and Area 2) only has a detailed map of its own roads and a summary of roads in the other districts. When a new road is built in Suburb East, only that district's office updates its detailed map. It sends a summary update (a type 3 LSA) to the central office, which then forwards that summary to Suburb West. Suburb West updates its map with the new road as a 'summary entry' (not the full detail). This mirrors OSPF multi-area: Area 0 (backbone) carries all inter-area routes, while non-backbone areas only know their own internal routes and summaries of other areas. The key is that detailed link-state information stays within each area, reducing the size of LSDBs and speeding up convergence. If the central office fails, the city is partitioned; similarly, OSPF requires all areas to connect to Area 0.
What Is OSPF Multi-Area?
OSPF (Open Shortest Path First) is a link-state routing protocol that builds a complete topology map (link-state database, LSDB) of the network. In a single-area design, all routers share the same LSDB, which can become huge in large networks — consuming CPU and memory and slowing convergence. Multi-area OSPF divides the network into areas, with Area 0 (backbone) at the center. Routers inside an area have detailed LSDBs for that area only; they receive only summary routes (type 3 LSAs) from other areas. This reduces LSDB size and limits the blast radius of topology changes.
OSPF Router Types
Internal Router (IR): All interfaces belong to the same area.
Area Border Router (ABR): Has interfaces in at least two areas, one of which must be Area 0. ABRs generate type 3 LSAs (summary LSAs) to advertise routes between areas.
Backbone Router: Has at least one interface in Area 0.
Autonomous System Boundary Router (ASBR): Redistributes external routes into OSPF (not required for basic multi-area).
How Multi-Area OSPF Works – Step by Step
Neighbor Discovery: Routers send Hello packets on all OSPF-enabled interfaces. Neighbors become adjacent after exchanging Hello/DBD packets and reaching Full state.
LSA Flooding Within Area: Each router floods its own link-state advertisements (LSAs) within its area. For example, a router in Area 1 floods type 1 Router LSAs and type 2 Network LSAs (if DR/BDR exist) only within Area 1.
ABR Generates Type 3 LSAs: The ABR (connected to Area 0 and Area 1) examines its LSDB for Area 1 and creates type 3 summary LSAs for each network in Area 1. It floods these into Area 0.
Backbone Flooding: Routers in Area 0 receive type 3 LSAs. The ABR for Area 2 also receives them and floods them into Area 2 as type 3 LSAs.
Route Calculation: Each router runs SPF on its area LSDB plus received type 3 LSAs to build the routing table. Inter-area routes appear as O IA (OSPF Inter-Area) routes with metric equal to cost to the ABR plus the advertised metric.
Key LSAs in Multi-Area
Type 1 (Router LSA): Describes a router's interfaces and neighbors within an area. Flooded only within the originating area.
Type 2 (Network LSA): Generated by the Designated Router (DR) on a broadcast segment. Also area-scoped.
Type 3 (Summary LSA): Generated by ABRs to advertise networks from one area to another. Contains the network prefix, mask, and metric.
Type 4 (ASBR Summary LSA): Used when redistributing external routes, not needed for basic multi-area.
Type 5 (External LSA): Generated by ASBRs for external routes.
OSPF Area Rules
All areas must connect to Area 0 (physically or via virtual link).
Area 0 must be contiguous.
A router can belong to multiple areas (ABR).
Inter-area routes must traverse Area 0.
Default Timers and Values
Hello Interval: 10 seconds (broadcast), 30 seconds (NBMA)
Dead Interval: 40 seconds (broadcast), 120 seconds (NBMA)
OSPF Metric: cost = 10^8 / bandwidth (default reference bandwidth 100 Mbps)
OSPF Router ID: Highest loopback IP, then highest active interface IP; can be manually set.
IOS CLI Verification Commands
show ip ospf neighbor
show ip ospf interface
show ip ospf database
show ip route ospf
show ip protocolsExample output for show ip ospf neighbor:
Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 1 FULL/DR 00:00:35 10.0.12.1 GigabitEthernet0/0
2.2.2.2 1 FULL/BDR 00:00:38 10.0.23.2 GigabitEthernet0/1Example output for show ip route ospf:
O 10.0.1.0/24 [110/2] via 10.0.12.1, 00:12:34, GigabitEthernet0/0
O IA 192.168.2.0/24 [110/3] via 10.0.12.1, 00:10:22, GigabitEthernet0/0Interaction with Other Protocols
Multi-area OSPF works with any other routing protocol via redistribution, but that requires an ASBR. Within OSPF, it interacts with virtual links (used when an area cannot physically connect to Area 0), stub areas, and NSSA.
Set up IP addressing and loopbacks
Configure IP addresses on all interfaces and assign loopback addresses for OSPF router IDs. For this lab, we use three routers: R1 (Area 0), R2 (ABR, Area 0 and Area 1), R3 (Area 1). Use the following commands on each router: ``` ! R1 interface GigabitEthernet0/0 ip address 10.0.12.1 255.255.255.0 no shutdown interface Loopback0 ip address 1.1.1.1 255.255.255.255 ! R2 interface GigabitEthernet0/0 ip address 10.0.12.2 255.255.255.0 no shutdown interface GigabitEthernet0/1 ip address 10.0.23.2 255.255.255.0 no shutdown interface Loopback0 ip address 2.2.2.2 255.255.255.255 ! R3 interface GigabitEthernet0/1 ip address 10.0.23.3 255.255.255.0 no shutdown interface Loopback0 ip address 3.3.3.3 255.255.255.255 ```
Enable OSPF and assign router IDs
Enter OSPF configuration mode and set the router ID manually to avoid confusion. Use the `router-id` command. Then enable OSPF on the interfaces using `network` statements under the OSPF process. The router ID must be unique across the OSPF domain. ``` ! R1 router ospf 1 router-id 1.1.1.1 network 10.0.12.0 0.0.0.255 area 0 network 1.1.1.1 0.0.0.0 area 0 ! R2 router ospf 1 router-id 2.2.2.2 network 10.0.12.0 0.0.0.255 area 0 network 10.0.23.0 0.0.0.255 area 1 network 2.2.2.2 0.0.0.0 area 0 ! R3 router ospf 1 router-id 3.3.3.3 network 10.0.23.0 0.0.0.255 area 1 network 3.3.3.3 0.0.0.0 area 1 ```
Verify OSPF neighbor relationships
Use `show ip ospf neighbor` to confirm that each router has formed full adjacency with its neighbors. R1 should see R2, R2 should see both R1 and R3, and R3 should see R2. The state should be FULL/DR or FULL/BDR. If not, check Hello/Dead timers, network type, and interface IP/subnet mask. ``` R1# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 2.2.2.2 1 FULL/DR 00:00:35 10.0.12.2 GigabitEthernet0/0 R2# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 1.1.1.1 1 FULL/BDR 00:00:38 10.0.12.1 GigabitEthernet0/0 3.3.3.3 1 FULL/DR 00:00:32 10.0.23.3 GigabitEthernet0/1 R3# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 2.2.2.2 1 FULL/BDR 00:00:36 10.0.23.2 GigabitEthernet0/1 ```
Examine OSPF LSDB for type 3 LSAs
On R1, run `show ip ospf database` to see the LSDB. You should see Router LSAs (type 1) and Network LSAs (type 2) for Area 0, and Summary LSAs (type 3) for routes from Area 1. On R3, you should see type 3 LSAs for Area 0 networks. This confirms that the ABR (R2) is generating summary LSAs. ``` R1# show ip ospf database OSPF Router with ID (1.1.1.1) (Process ID 1) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 1.1.1.1 1.1.1.1 100 0x80000003 0x00A1B2 1 2.2.2.2 2.2.2.2 95 0x80000004 0x00C3D4 2 Summary Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 10.0.23.0 2.2.2.2 80 0x80000001 0x00E5F6 3.3.3.3 2.2.2.2 80 0x80000001 0x00G7H8 ```
Verify routing tables for inter-area routes
Check the routing table on R1 and R3. R1 should have an O IA route to the 10.0.23.0/24 network and to R3's loopback (3.3.3.3/32). R3 should have O IA routes to 10.0.12.0/24 and R1's loopback. The metric shown is the total cost from the local router to the destination via the ABR. ``` R1# show ip route ospf 10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks O IA 10.0.23.0/24 [110/2] via 10.0.12.2, 00:15:00, GigabitEthernet0/0 O IA 3.3.3.3/32 [110/3] via 10.0.12.2, 00:15:00, GigabitEthernet0/0 R3# show ip route ospf 10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks O IA 10.0.12.0/24 [110/2] via 10.0.23.2, 00:14:45, GigabitEthernet0/1 O IA 1.1.1.1/32 [110/3] via 10.0.23.2, 00:14:45, GigabitEthernet0/1 ```
Test connectivity and troubleshoot
Ping from R1 to R3's loopback (3.3.3.3) to verify end-to-end connectivity. If ping fails, check OSPF neighbor states, routing tables, and ACLs. Use `debug ip ospf adj` to see adjacency formation issues. Common problems: mismatched area IDs, wrong network statements, or passive interfaces. Also verify that the ABR has both areas configured correctly. ``` R1# ping 3.3.3.3 source 1.1.1.1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 3.3.3.3, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms ```
In a large enterprise with hundreds of routers, a single-area OSPF design would cause every router to maintain an LSDB containing every link in the network. This consumes significant memory and CPU, and any topology change triggers SPF recalculation across the entire domain. Multi-area OSPF solves this by partitioning the network into areas. For example, a university campus might have Area 0 for the core backbone, Area 1 for the engineering building, Area 2 for the library, and Area 3 for the dormitories. Each building's internal routers only know their own area's topology in detail; they receive summary routes for other buildings via the ABR. This reduces LSDB size and isolates failures — a flapping link in the library does not cause SPF runs in the engineering building.
Another common scenario is a multi-site WAN. Each site can be its own area, with Area 0 connecting the sites via high-speed WAN links. The ABR at each site summarizes the site's internal networks into Area 0. This keeps WAN link-state updates minimal and speeds convergence. In production, network engineers must carefully choose area boundaries to optimize performance. For example, placing too many routers in a single area defeats the purpose of multi-area. Also, all areas must connect to Area 0; if a new area cannot physically connect, a virtual link can be used, but this adds complexity.
Misconfiguration can lead to routing black holes. A common mistake is forgetting to configure the network statement for the link between ABR and a non-backbone area, causing that area to be isolated. Another is mismatched area IDs on the same link — OSPF will not form adjacency if areas differ. Also, if an ABR is not configured with area 0, it cannot generate type 3 LSAs, and inter-area routing fails. In production, always verify with show ip ospf neighbor and show ip route ospf after changes.
The CCNA 200-301 exam objective 3.4 requires you to configure and verify multi-area OSPF. You must know the router types, LSA types (especially type 1, 2, 3), and how inter-area routes appear in the routing table (O IA). The exam often presents a topology diagram and asks which router is an ABR, or which LSA type is used to advertise a route between areas. Another common question: 'Which area must all other areas connect to?' Answer: Area 0.
Common wrong answers: - Confusing ABR with ASBR: An ABR connects areas; an ASBR connects OSPF to another routing protocol. The exam may show a router redistributing EIGRP into OSPF — that is an ASBR, not necessarily an ABR. - Thinking that type 3 LSAs contain full topology: They are summary LSAs with prefix, mask, and metric, not link details. - Assuming that inter-area routes have the same metric as intra-area: Inter-area routes have metric = cost to ABR + advertised metric, which is typically higher than intra-area routes. - Forgetting that OSPF requires all areas to connect to Area 0: A question may describe a network with Area 1 and Area 2 connected via a router not in Area 0 — that router would not be able to exchange inter-area routes without a virtual link.
Calculation traps: The metric for O IA routes is cumulative. For example, if R1 to ABR cost is 10, and ABR advertises a network with metric 20, the total metric is 30. The exam may ask you to compute the metric for a given path.
Decision rule for scenario questions: If a router has interfaces in multiple areas, it is an ABR only if one of those areas is Area 0. If a route is learned via OSPF and shows 'O IA', it originated in a different area. Use show ip ospf database summary to see type 3 LSAs.
OSPF multi-area reduces LSDB size and limits SPF calculations to within an area.
Area 0 (backbone) must be contiguous and all other areas must connect to it.
ABRs generate type 3 summary LSAs to advertise routes between areas.
Inter-area routes appear as 'O IA' in the routing table with cumulative metric.
Router ID must be unique; can be set manually with 'router-id' command.
Use 'show ip ospf neighbor' to verify adjacencies; state must be FULL.
Common misconfiguration: mismatched area IDs on a link prevent adjacency.
These come up on the exam all the time. Here's how to tell them apart.
Single-Area OSPF
All routers share the same LSDB.
Any topology change triggers SPF in all routers.
No ABRs; all routers are internal.
Maximum network size limited by CPU/memory.
Simpler configuration.
Multi-Area OSPF
Each area has its own LSDB; ABRs summarize between areas.
Topology changes are contained within an area.
ABRs connect areas and generate type 3 LSAs.
Scales to larger networks.
More complex configuration, but better performance.
Mistake
An ABR must have at least two interfaces in different areas.
Correct
An ABR must have at least one interface in Area 0 and at least one in another area. Having two non-backbone areas does not make a router an ABR; it must connect to Area 0.
Candidates often think any router with multiple areas is an ABR, but the backbone requirement is key.
Mistake
Type 3 LSAs contain detailed link-state information about the originating area.
Correct
Type 3 LSAs are summary LSAs that contain only the prefix, mask, and metric. They do not include topology details.
Confusing type 3 with type 1 or type 2 LSAs, which do contain detailed link info.
Mistake
Inter-area routes have the same metric as intra-area routes.
Correct
Inter-area routes have a metric equal to the cost from the local router to the ABR plus the cost advertised in the type 3 LSA. This is typically higher than intra-area metrics.
Candidates assume all OSPF routes are equal, but inter-area routes are always via an ABR.
Mistake
A router can belong to multiple non-backbone areas without connecting to Area 0.
Correct
OSPF requires that all areas connect to Area 0. A router with interfaces in Area 1 and Area 2 but not Area 0 cannot exchange inter-area routes between them without a virtual link.
Misunderstanding the requirement for all areas to be attached to Area 0.
Reveal each answer, then mark whether you got it right. Score 60%+ to unlock the next chapter.
Enter OSPF process configuration with 'router ospf [process-id]'. Use 'network [prefix] [wildcard] area [area-id]' to enable OSPF on interfaces. For multi-area, assign different area IDs on different interfaces. Ensure one area is Area 0. For example, on an ABR: 'network 10.0.1.0 0.0.0.255 area 0' and 'network 10.0.2.0 0.0.0.255 area 1'. Verify with 'show ip ospf neighbor' and 'show ip route ospf'.
In single-area OSPF, all routers are in the same area and share the same LSDB. This works for small networks but does not scale. Multi-area divides the network into areas, with Area 0 as the backbone. ABRs summarize routes between areas, reducing LSDB size and isolating topology changes. Multi-area requires more configuration but is essential for larger networks.
OSPF uses a two-level hierarchy: Area 0 is the backbone. All inter-area traffic must traverse Area 0 to prevent routing loops. If an area cannot physically connect to Area 0, a virtual link can be used to create a logical connection through another area. Without connection to Area 0, inter-area routing will not work.
Use 'show ip ospf neighbor' to see adjacencies and ensure routers in different areas are adjacent. Use 'show ip route ospf' to see inter-area routes (O IA). Use 'show ip ospf database' to see LSAs; type 3 LSAs indicate summary routes. On an ABR, 'show ip ospf' shows the areas it is connected to. Also check 'show ip protocols' for OSPF process details.
Common mistakes include: (1) Not configuring Area 0 on the ABR, (2) Using the same area ID on both sides of a link, (3) Forgetting to include the link network in the OSPF process, (4) Mismatched Hello/Dead timers, (5) Making an interface passive accidentally. Always verify with 'show ip ospf interface' and 'show ip ospf neighbor'.
No. By definition, an ABR must have at least one interface in Area 0. If a router has interfaces in multiple non-backbone areas, it is not an ABR and cannot generate type 3 LSAs. It would be considered an internal router for each area, but inter-area routing would not function correctly.
The metric for an inter-area route is the cost from the local router to the ABR plus the cost advertised in the type 3 LSA (which is the cost from the ABR to the destination network). This is cumulative and typically higher than intra-area routes. The metric is displayed in the routing table as the second number in brackets, e.g., [110/20].
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