Cisco CCNP Enterprise (ENCOR 350-401) · lesson 1 of 12 · 35 min
Why Multi-Area OSPF?
Single-area OSPF works fine for small networks, but as a network grows, every router must store the entire link-state database (LSDB), run the Dijkstra SPF algorithm against it, and flood every LSA to every other router. In a network with 500 routers, that is a massive CPU and memory burden — and every topology change triggers a full SPF recalculation across the entire area. Multi-area OSPF solves this by dividing the network into smaller areas. Each area maintains its own LSDB. Routers inside an area only know the detailed topology of their own area; they receive summarized information about other areas. This dramatically reduces LSDB size, limits LSA flooding scope, and confines SPF recalculations to the affected area. Think of a large country divided into states. Each state manages its own internal roads in detail. When you travel between states, you only need to know the interstate highways — not every local street in every other state. Multi-area OSPF works the same way: detailed maps inside each area, summarized routes between areas.
OSPF Area Types
The ENCOR exam loves stub/NSSA distinctions. Remember: stub blocks Type 5, totally stubby blocks Type 3 AND Type 5, NSSA converts Type 5 to Type 7 at the ABR.
OSPF Router Roles
OSPF LSA Types — The Core of the Exam
LSAs (Link-State Advertisements) are the packets OSPF uses to describe the network topology. Each LSA type carries different information and has different flooding scope. Mastering LSA types is essential for the ENCOR exam. How LSAs work together When a router in Area 2 wants to reach an external route (e.g., an internet prefix redistributed from BGP): it uses a Type 5 LSA to find the prefix, a Type 4 LSA to find the ASBR’s router ID, and Type 1/2 LSAs to find the path to the ABR. All four LSA types work together to build the complete routing picture.
OSPF Packet Types
OSPF Neighbor States
OSPF adjacency formation follows a strict state machine. Understanding each state helps you troubleshoot why two routers are stuck and not exchanging routes. Stuck in 2-Way is normal for DROther routers — they only reach Full with DR and BDR. Stuck in ExStart/Exchange usually means MTU mismatch. Stuck in Loading means LSA corruption or retransmit issues.
OSPF Hello Parameters — Must Match
Worked Scenario: Designing a Multi-Area OSPF Network
Scenario You are designing OSPF for a company with headquarters (HQ), three branch offices, and two data centers. The HQ core switches will be ABRs. Branch offices have a single exit point. Data centers redistribute BGP routes into OSPF. Step 1 — Area 0 (Backbone): Place HQ core switches and the links between them in Area 0. All inter-area traffic transits here. Step 2 — Branch areas: Each branch gets its own area (Area 1, 2, 3). Since each branch has only one exit, configure them as totally stubby areas — this gives each branch router a single default route instead of a full routing table. Step 3 — Data center areas: Data centers redistribute BGP, so they need ASBRs. Configure them as NSSA areas so external routes can enter via Type 7 LSAs, which the ABR converts to Type 5 for the rest of the domain. Step 4 — Summarization: Configure the ABRs to summarize each area’s prefix range into Area 0. This keeps the backbone LSDB small and prevents individual subnet flaps from propagating everywhere.
Flashcard Drill
Type 1 LSA Router LSA — every router generates one, flooded within its area only, describes the router’s own links. Type 3 LSA Summary LSA — generated by ABR, carries inter-area prefix information into other areas. Type 5 LSA AS External LSA — generated by ASBR, flooded throughout entire OSPF domain (except stubs). Type 7 LSA NSSA External LSA — used inside NSSA areas instead of Type 5; ABR converts it to Type 5 at the boundary. ABR Area Border Router — connects two or more OSPF areas, maintains separate LSDB per area. ASBR AS Boundary Router — redistributes external routes into OSPF, generates Type 5/7 LSAs. Totally stubby area Cisco extension — blocks Type 3 and Type 5 LSAs, only a default route enters. Smallest possible LSDB. NSSA Not-So-Stubby Area — stub area that also has an ASBR; uses Type 7 LSAs instead of Type 5. DR/BDR election On multi-access networks, the router with the highest OSPF priority wins DR (default priority 1). Tie broken by highest router ID. Full state OSPF adjacency is complete — LSDB is synchronized and routes are being exchanged. ExStart stuck Usually caused by MTU mismatch between neighbors — check ip mtu and ip ospf mtu-ignore. SPF algorithm Dijkstra’s Shortest Path First — OSPF runs this against the LSDB to build the routing table. Runs per-area.
Knowledge Check — 15 Questions
OSPF Multi-Area & LSA Types
Which LSA type is generated by an ABR to advertise inter-area routes?
- Type 1
- Type 2
- Type 3
- Type 5
Answer: Type 3
Type 3 Summary LSAs are generated by ABRs to carry prefix information between areas.
A router in a totally stubby area needs to reach an external internet prefix. How does it do this?
- It uses a Type 5 LSA
- It uses a Type 3 LSA for the specific prefix
- It uses the default route injected by the ABR
- It cannot reach external prefixes
Answer: It uses the default route injected by the ABR
Totally stubby areas block both Type 3 and Type 5 LSAs. The ABR injects a single default route, which the internal router uses for all inter-area and external destinations.
Which OSPF neighbor state indicates that the LSDB is fully synchronized?
- 2-Way
- Exchange
- Loading
- Full
Answer: Full
Full state means the LSDB is completely synchronized and the adjacency is established.
OSPF LSAs generated by an ASBR inside an NSSA area are called Type _____ LSAs.
Answer: 7
Type 7 NSSA External LSAs are used inside NSSA areas. The ABR converts them to Type 5 when flooding to the rest of the OSPF domain.
Two OSPF routers are stuck in ExStart state. What is the most likely cause?
- Mismatched area IDs
- MTU mismatch
- Different Hello intervals
- Duplicate router IDs
Answer: MTU mismatch
MTU mismatch is the classic cause of ExStart/Exchange stuck state. DBD packets exceed the MTU and are dropped.
Which area type blocks Type 5 LSAs but allows Type 3 LSAs?
- Totally stubby
- NSSA
- Stub
- Backbone
Answer: Stub
A standard stub area blocks Type 5 AS External LSAs but still allows Type 3 Summary LSAs from the ABR.
A DROther router reaches Full state with all other routers in the segment.
- True
- False
Answer: False
DROther routers only reach Full state with the DR and BDR. They remain in 2-Way state with other DROther routers.
Which OSPF packet type is used to confirm receipt of LSUs?
- Hello
- DBD
- LSR
- LSAck
Answer: LSAck
LSAck (Type 5) acknowledges received LSUs, making OSPF reliable.
An ABR generates a Type 4 LSA. What does it describe?
- The ABR’s own interfaces
- The location of an ASBR
- A summary of inter-area prefixes
- External routes from BGP
Answer: The location of an ASBR
Type 4 ASBR Summary LSAs tell routers in other areas where to find the ASBR (its router ID), so they can reach Type 5 LSAs.
What is the primary benefit of configuring OSPF route summarization on an ABR?
- It increases LSDB size
- It prevents SPF from running
- It reduces LSDB size and hides topology instability
- It converts Type 5 to Type 7
Answer: It reduces LSDB size and hides topology instability
Summarization on ABRs reduces the number of LSAs flooded into Area 0 and prevents individual subnet flaps from triggering SPF in other areas.
Which OSPF Hello parameter mismatch will NOT prevent neighbor adjacency from forming?
- Hello interval
- Dead interval
- Area ID
- MTU
Answer: MTU
MTU mismatch does not prevent Hello exchange or the Init/2-Way states, but it will cause the adjacency to get stuck in ExStart/Exchange when DBD packets are exchanged.
The OSPF router with the highest _____ wins the DR election on a multi-access segment (assuming equal priority).
Answer: router ID
When OSPF priority is equal, the highest router ID wins the DR election. Router ID is typically the highest loopback IP.
A router has interfaces in Area 0 and Area 1. What OSPF role does it play?
- Internal Router
- ASBR
- ABR
- DR
Answer: ABR
A router with interfaces in two or more OSPF areas is an Area Border Router (ABR). It maintains a separate LSDB for each area.
Which LSA type is flooded throughout the entire OSPF domain except stub areas?
- Type 1
- Type 2
- Type 3
- Type 5
Answer: Type 5
Type 5 AS External LSAs are flooded domain-wide except into stub and totally stubby areas.
All OSPF areas must connect to Area 0 either directly or via a virtual link.
- True
- False
Answer: True
Area 0 is the backbone. All non-backbone areas must have a direct or virtual-link connection to Area 0 for inter-area routing to work correctly.
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