A branch router does not always need every external route in the OSPF domain. The interview question is: what is the difference between an OSPF stub area, a totally stubby area and an NSSA?
Short answer: a stub area blocks Type 5 external LSAs and uses an ABR-advertised default for external destinations; a totally stubby area also suppresses specific inter-area routes; an NSSA still blocks incoming Type 5 LSAs but allows local redistribution using Type 7 LSAs.[6][1]
OSPF area types: comparison table
This article covers OSPFv2 for IPv4. Configuration examples use Cisco IOS-style syntax; check your platform and software release before applying them. The table describes the LSAs available to a router internal to the area, not every database section visible on an ABR.
| Area type | Local Type 1/2 LSAs | Specific inter-area Type 3 LSAs | Type 4/5 LSAs from outside | Local redistribution as Type 7 | Default route |
|---|---|---|---|---|---|
| Normal area | Yes, where applicable | Allowed | Allowed | No | Not created just by choosing a normal area |
| Stub | Yes | Allowed | Blocked | No | ABR advertises Type 3 default |
| Totally stubby | Yes | Suppressed, except default | Blocked | No | ABR advertises Type 3 default |
| NSSA | Yes | Allowed | Blocked | Yes | Do not assume an automatic default |
| Totally NSSA | Yes | Suppressed, except default | Blocked | Yes | Cisco no-summary causes Type 3 default |
Stub-area default routing and external-LSA exclusion are defined in RFC 2328; Cisco documents the NSSA variants, Type 7 redistribution and their different default-origination behavior.[6][1]
Do not confuse an LSA with a route: a Type 3 LSA describes inter-area reachability, including a possible default, while a Type 5 or Type 7 LSA describes external reachability.[6][1]
Stub area: fewer external details, not fewer local routers
A stub area retains its intra-area topology and can retain specific routes to other OSPF areas; external destinations are reached through a summary default advertised by an ABR.[6]
An internal stub router therefore does not need the Type 5 advertisement for every redistributed prefix elsewhere in the domain.[6]
For an original branch example, imagine branch-router — ABR — area 0 — external-edge. If the branch has no more-specific route to an external destination, its default sends the packet toward the ABR; the ABR then needs a usable onward route. This is an illustrative forwarding scenario, not a measured lab result.
A stub area cannot contain an internal ASBR and cannot serve as the transit area for an OSPF virtual link.[6]
Design takeaway: consider a stub when the branch does not redistribute routes and does not need destination-specific external exit selection. Multiple ABRs are possible; a stub area is not defined by having exactly one exit.[6]
Totally stubby area: suppress specific inter-area routes too
“Totally stubby” is the common Cisco term for a stub area where the ABR suppresses specific inter-area summaries and leaves the default; RFC 2328 also allows summary-LSA import to be controlled for a stub area.[6]
The practical distinction is stub = keep inter-area detail; totally stubby = use a default instead of that detail. Local area topology does not disappear.[6]
Here are minimal IOS-style area-policy fragments for an illustrative area 10. Interfaces, addressing and area assignments are assumed to exist already. These are alternatives, not commands to stack together.
! Ordinary stub: configure on every router participating in area 10
router ospf 10
area 10 stub
! Totally stubby: use this area policy on every area 10 ABR
router ospf 10
area 10 stub no-summary
! Internal routers of that totally stubby area still use:
router ospf 10
area 10 stub
In this design, put the summary-suppression policy on every ABR feeding the area; otherwise another ABR can still introduce the specific inter-area routes you intended to remove. Treat an area-type change as a coordinated maintenance change, not an isolated command experiment.
NSSA: allow redistribution inside the branch
An NSSA solves a different problem: the branch needs to inject a locally learned external route while still excluding Type 5 LSAs coming from the rest of OSPF.[1]
The internal ASBR originates a Type 7 LSA inside the NSSA, and an NSSA ABR can translate an eligible Type 7 into a Type 5 for advertisement into the wider OSPF domain.[1]
The control-plane flow is:
Local external prefix at branch ASBR
-> redistribution policy permits the prefix
-> Type 7 LSA inside the NSSA
-> NSSA ABR translates eligible advertisement
-> Type 5 LSA into normal OSPF areas
This flow is an original schematic of the redistribution and translation process, not a packet capture.[1]
Type 7 is area-scoped; it is not simply flooded unchanged into area 0. Also, not every Type 7 must become Type 5: the P-bit and translation eligibility matter.[1]
For a plain NSSA, configure the NSSA area type on all participating routers:[1]
router ospf 10
area 10 nssa
This alone does not redistribute a static route or another protocol. Add an explicitly filtered redistribution policy only if the design requires it, and verify the exact prefixes rather than enabling broad redistribution as a shortcut.
The default-route trap in NSSA
On the Cisco behavior documented here, a plain NSSA ABR does not automatically originate a default merely because the area is configured as NSSA.[1][4]
To advertise a Type 7 default from the NSSA ABR, Cisco documents this area-specific command:[1][4]
router ospf 10
area 10 nssa default-information-originate
By contrast, this ABR configuration makes the area totally NSSA, suppresses specific Type 3 summaries and generates a Type 3 default; internal routers remain configured with area 10 nssa.[1]
router ospf 10
area 10 nssa no-summary
Do not mix these two mechanisms when explaining the result: one originates a Type 7 default; the other supplies a summary default while suppressing inter-area detail.[1]
Cisco also documents that the NSSA ABR can originate the Type 7 default without having a default in its own routing table, whereas an NSSA ASBR's origination has an existing non-OSPF default requirement.[1][4]
Operational recommendation: test actual upstream reachability and failure behavior. Seeing 0.0.0.0/0 on the branch is not evidence that the upstream router can deliver the traffic.
Troubleshooting checklist: neighbors, database, route, forwarding
Use this order in a lab or maintenance window:
- Check area-type agreement. All routers must agree on stub/NSSA membership; the OSPF Hello options carry capability information, and a stub E-bit mismatch causes Hello rejection.[6][1]
- Identify where you are looking. On an ABR, inspect the relevant area database section; seeing external information associated with its normal-area participation does not mean Type 5 LSAs were flooded inside the stub area.
- Inspect the database before the routing table. Cisco provides
show ip ospf database,show ip ospf database nssa-externalandshow ip routeto distinguish received LSAs from installed routes.[4] - Check default origination explicitly. For plain NSSA, inspect the ABR policy instead of assuming stub-style automatic defaults.[1][4]
- If a redistributed prefix is missing outside the NSSA, inspect the redistribution filter, the Type 7 advertisement and its translation eligibility.[1]
- Test the data plane. Check the ABR's onward route, return routing and relevant security policy before calling the design healthy.
Useful IOS-style read-only commands include:
show ip ospf
show ip ospf neighbor
show ip ospf database
show ip ospf database summary
show ip ospf database nssa-external
show ip route ospf
show ip route 0.0.0.0
No router output is reproduced here: these are suggested verification commands, not claims that a live network was tested.
Common interview pitfalls
- “Totally stubby means only a default exists.” The area still needs its local topology and intra-area routes; it is outside route detail that is reduced.[6]
- “NSSA accepts external Type 5 LSAs.” No: its local redistribution uses Type 7 instead.[1]
- “Every NSSA has a default automatically.” Not for a plain NSSA under the documented Cisco behavior.[1][4]
- “No-summary prevents NSSA redistribution.” It suppresses inter-area summaries, not the NSSA's Type 7 capability.[1]
- “Stub is a one-router or one-exit topology.” Stub is an area policy; RFC 2328 allows cases where exit selection need not be external-destination-specific.[6]
A 30-second interview answer
“A stub area removes Type 5 external LSAs and uses an ABR default for external destinations. A totally stubby area also removes specific inter-area summaries. If I must redistribute a route inside that area, I use NSSA: the ASBR originates Type 7, and eligible advertisements are translated to Type 5 at the ABR. I then verify default-route behavior explicitly, because plain NSSA and totally NSSA do not originate defaults in the same way.”[6][1]
Related reading
- OSPF LSA Types: Practical Guide for Network Engineers
- OSPF Hello Packets: Timers, Neighbors and Troubleshooting Checks
- OSPF Stuck in ExStart or Exchange: MTU and DBD Troubleshooting
Summary and interview practice
Remember the choice as external detail, inter-area detail, local redistribution. Those three questions explain the difference more clearly than memorizing area names.
Preparing for a network engineering interview? Draw one ABR and one branch router, choose an area type, and explain which LSA carries the default and how a locally redistributed prefix reaches area 0. Then validate your explanation in an isolated lab.
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