OSPF stub areas and totally stubby areas are essential tools for reducing the size of the Link State Database (LSDB) and the routing table in large OSPF networks. On the CCNA 200-301 exam (objective 3.4), you must understand how these area types filter Type 3, Type 4, and Type 5 LSAs, and how default routes are injected. In real networks, stub areas prevent unnecessary routing information from flooding into low-memory routers at remote sites, improving stability and convergence.
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Imagine a large corporation with a central headquarters (the backbone area 0) and several remote branch offices (stub areas). The central library contains every book (all routes) in the company's catalog, including inter-company publications (external routes). Each branch office has its own small library that only needs books relevant to its local department. To avoid wasting shelf space, the branch library subscribes to a filtered catalog service: it receives only internal company books (inter-area routes) and a single 'general reference' book (default route) that says "if you don't find it here, ask the main library." This is exactly what a standard stub area does — it blocks Type 5 LSAs (external routes) and relies on a default route to reach external destinations. Now, a totally stubby area is even stricter: it blocks both Type 5 and Type 4 LSAs, and also blocks Type 3 LSAs (inter-area routes) except for the default route. In our analogy, the branch library only gets the 'general reference' book and no other inter-department books. This drastically reduces the number of entries in the branch's routing table, just as a totally stubby area reduces the LSDB size to only Type 1, Type 2, and a single Type 3 default. The mechanism works because the Area Border Router (ABR) at the area boundary performs the filtering; in the library analogy, the ABR is the librarian who decides which books to send to the branch. This filtering is configured using the area stub or area stub no-summary command on the ABR and all routers in the area.
What Are OSPF Stub Areas and Totally Stubby Areas?
OSPF divides the network into areas to limit the scope of Link State Advertisements (LSAs). In a standard area, routers receive all types of LSAs: Type 1 (Router), Type 2 (Network), Type 3 (Summary/Inter-area), Type 4 (ASBR Summary), and Type 5 (External). This can overwhelm routers with limited memory or CPU, especially at remote sites. Stub areas solve this by blocking Type 5 LSAs (external routes) from entering the area. The ABR automatically injects a default route (0.0.0.0/0) as a Type 3 LSA so that routers in the stub area can reach external destinations by sending traffic to the ABR. A totally stubby area (also called a 'stub no-summary' area) goes further by also blocking Type 3 and Type 4 LSAs, except for the default route. This results in the smallest possible LSDB and routing table within the area.
How It Works Step by Step
Configuration: On each router in the area (including the ABR), you configure the area <area-id> stub command. For a totally stubby area, you add the no-summary keyword only on the ABR. This command puts the router into stub mode, which means it will not accept Type 5 LSAs and will not originate Type 5 LSAs. The ABR then automatically generates a default Type 3 LSA into the stub area.
LSA Flooding Control: When a router in a stub area receives a Type 5 LSA, it drops it. Similarly, a totally stubby area router drops Type 3, Type 4, and Type 5 LSAs. The ABR filters these LSAs at the area boundary. For totally stubby, the ABR does not forward any Type 3 LSAs (including inter-area routes) except the default route it generates.
Default Route Injection: The ABR creates a Type 3 LSA with link-state ID 0.0.0.0, network mask 0.0.0.0, and metric 1 (by default). This LSA is flooded throughout the stub area. All routers in the area install a default route via the ABR.
Routing Table Impact: In a standard area, a router may have hundreds of routes. In a stub area, only intra-area (O) and inter-area (O IA) routes appear, plus the default route (O*E2 or O*IA). In a totally stubby area, only intra-area routes and the default route exist — no inter-area routes at all.
Key States, Timers, and Defaults
Command: area <area-id> stub [no-summary]
Default metric for default route: 1 (cost). Can be changed with area <area-id> default-cost <cost>.
Restriction: The backbone area (0) and area 0 cannot be stub. Also, virtual-link transit areas cannot be stub.
Consistency: All routers in a stub area must agree on the stub flag in Hello packets. If one router is configured as stub and another is not, the neighbor relationship will not form (stuck in EXSTART/EXCHANGE).
LSA types allowed in stub area: Type 1, Type 2, Type 3 (except in totally stubby), and Type 3 default.
LSA types allowed in totally stubby area: Type 1, Type 2, and Type 3 default only.
IOS CLI Verification Commands
To verify stub area configuration and operation:
show ip ospf interface [interface]Example output for a stub interface:
FastEthernet0/0 is up, line protocol is up
Internet Address 10.0.1.1/24, Area 1
Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 1.1.1.1, Interface address 10.0.1.1
Backup Designated router (ID) 2.2.2.2, Interface address 10.0.1.2
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:02
Supports Link-local Signaling (LLS)
Cisco NSF helper support enabled
IETF NSF helper support enabled
Index 1/1/1, flood queue length 0
Next 0x0(0)/0x0(0)/0x0(0)
Last flood scan length is 0, maximum is 25
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 2.2.2.2 (Backup Designated Router)
Suppress hello for 0 neighbor(s)
Area 1 is a STUB areaNote the line "Area 1 is a STUB area".
show ip ospf databaseThis shows the LSDB. In a stub area, you will not see Type 5 LSAs. In a totally stubby area, you will see only Type 1, Type 2, and a Type 3 default LSA.
show ip route ospfShows the routing table. Look for the default route (O*IA 0.0.0.0/0) in stub/totally stubby areas.
Interaction with Related Protocols
Stub areas interact with OSPF's LSA types and the SPF algorithm. By reducing the number of LSAs, the SPF calculation is faster and the LSDB is smaller. However, stub areas can cause suboptimal routing if the default route is not carefully costed. Also, if an ASBR is inside a stub area, it cannot inject external routes because Type 5 LSAs are blocked. Therefore, redistribution must occur at the ABR or in a non-stub area.
Configure stub area on all routers
On every router in the stub area (including the ABR), enter OSPF configuration mode and issue the command `area <area-id> stub`. For example, `area 1 stub`. This sets the stub flag in Hello packets. All routers in the area must agree on this flag; otherwise, neighbor adjacency will not form. The ABR will automatically generate a default route into the area.
Configure totally stubby on ABR only
To create a totally stubby area, add the `no-summary` keyword only on the Area Border Router(s). For example: `area 1 stub no-summary`. This tells the ABR to block all Type 3 LSAs (including inter-area routes) from entering the area, except for the default route it generates. Internal routers still use the plain `area 1 stub` command — they do not need the `no-summary` keyword. The ABR will still send the default route.
Adjust default route cost (optional)
By default, the ABR advertises the default route with a metric of 1. You can change this cost using `area <area-id> default-cost <cost>` on the ABR. For example, `area 1 default-cost 10`. This influences which ABR a router prefers when multiple ABRs exist. A lower cost makes that ABR more attractive for default traffic.
Verify stub area status
Use `show ip ospf interface` to confirm the interface is in a stub area. Look for the line "Area X is a STUB area". Also use `show ip ospf database` to see which LSA types are present. In a stub area, you should see no Type 5 LSAs. In a totally stubby area, you should see only Type 1, Type 2, and a Type 3 default LSA (link-state ID 0.0.0.0).
Check routing table for default route
Use `show ip route ospf` to verify that the default route appears. In a stub area, the default route will show as `O*IA 0.0.0.0/0 [110/1] via <next-hop>`. The `O*IA` indicates an inter-area default route. In a totally stubby area, it will also be `O*IA`. Ensure the metric matches the configured default-cost.
Troubleshoot stub area mismatches
If routers do not form adjacency, check that all routers in the area have the `area stub` command. A common mistake is forgetting to configure stub on the ABR or on an internal router. Use `debug ip ospf hello` to see Hello packets and check the stub flag. Also verify that the area is not area 0 and that no virtual links traverse the area.
In enterprise networks, stub and totally stubby areas are commonly deployed at remote branch offices where routers have limited memory and CPU. For example, a retail chain with hundreds of stores might place each store's LAN into a totally stubby area. The store router only needs to know about its local subnets and a default route pointing to the regional headquarters. This drastically reduces the routing table size from thousands of entries to perhaps a dozen. The ABR at the regional office filters all inter-area and external routes, injecting only the default. This improves convergence time because the store router runs SPF only on a small LSDB.
Another scenario is a DMZ area where servers are located. Here, a standard stub area might be used because the DMZ needs to know about internal subnets (inter-area routes) but does not need external routes from the internet (which would be Type 5 LSAs). The DMZ router gets a default route to reach the internet via the firewall, which is redistributed into OSPF as an external route. By making the DMZ a stub area, the router avoids learning hundreds of internet routes via OSPF.
A common misconfiguration is making area 0 a stub area. The backbone area cannot be stub because it must carry all routes. Another mistake is using a totally stubby area when routers need to reach specific inter-area subnets via optimal paths; the default route forces all inter-area traffic through the ABR, which can create suboptimal routing or choke points. Network engineers must carefully plan which areas can be stub and adjust the default-cost to influence traffic flow. In large networks with multiple ABRs, the default route can be advertised with different costs to provide redundancy and load balancing.
The CCNA 200-301 exam tests your understanding of OSPF stub and totally stubby areas under objective 3.4 (Configure and verify OSPF). You must know:
The exact command syntax: area <area-id> stub and area <area-id> stub no-summary.
That the no-summary keyword is configured only on the ABR, not on internal routers.
Which LSA types are filtered: stub blocks Type 5; totally stubby blocks Type 3, Type 4, and Type 5.
That a default route is automatically injected by the ABR as a Type 3 LSA.
That all routers in the area must agree on the stub flag; otherwise, adjacency fails.
Common wrong answers:
1. "Stub areas block Type 3 LSAs." This is false; only totally stubby blocks Type 3. Stub areas allow Type 3 LSAs.
2. "The default route is injected as a Type 5 LSA." False; it is a Type 3 LSA (summary).
3. "You can configure stub on area 0." False; area 0 cannot be stub.
4. "The no-summary keyword is configured on all routers in the area." False; only on the ABR.
Exam questions often present a scenario with a routing table and ask which routes are present. For a totally stubby area, expect only O (intra-area) and O*IA (default). For a stub area, expect O, O IA (inter-area), and O*IA (default). No O E1/E2 routes.
A common trap: the exam may show a router in a stub area that has an O E2 route. This indicates a misconfiguration because external routes should not appear. Similarly, if a totally stubby area router shows an O IA route to a specific subnet, the configuration is wrong.
Decision rule: If the question says "small memory" or "minimize routing table", choose totally stubby. If it says "needs to know about other internal subnets but not external", choose stub.
Stub areas block Type 5 LSAs (external routes) and inject a default route as Type 3.
Totally stubby areas block Type 3, Type 4, and Type 5 LSAs, allowing only Type 1, Type 2, and a Type 3 default.
Command: `area <area-id> stub` on all routers; add `no-summary` only on ABR for totally stubby.
Default route metric is 1 by default; can be changed with `area <area-id> default-cost <cost>`.
Area 0 cannot be stub; virtual-link transit areas cannot be stub.
All routers in a stub area must be configured as stub; mismatch prevents adjacency.
Verify with `show ip ospf interface` (look for 'STUB area') and `show ip route ospf` (look for O*IA default).
These come up on the exam all the time. Here's how to tell them apart.
Standard Area
Allows Type 1, 2, 3, 4, 5 LSAs
Routing table includes O, O IA, O E1/E2
No automatic default route
Larger LSDB and routing table
Suitable for backbone and transit areas
Stub Area
Blocks Type 5 LSAs
Routing table includes O, O IA, O*IA (default)
ABR injects default route as Type 3
Smaller LSDB and routing table
Suitable for leaf areas without external connections
Stub Area
Blocks Type 5 only
Allows Type 3 (inter-area) routes
Routing table has O, O IA, O*IA
Default route injected by ABR
Still learns about other areas
Totally Stubby Area
Blocks Type 3, 4, and 5
Only Type 1, 2, and Type 3 default
Routing table has only O and O*IA
Default route injected by ABR
Minimal routing table; all inter-area traffic goes via ABR
Mistake
Stub areas block both Type 3 and Type 5 LSAs.
Correct
Stub areas block only Type 5 LSAs. Type 3 LSAs (inter-area routes) are allowed. Totally stubby areas block Type 3, Type 4, and Type 5.
Candidates confuse 'stub' with 'totally stubby' because both reduce routing information.
Mistake
The default route in a stub area is injected as a Type 5 external LSA.
Correct
The default route is injected as a Type 3 summary LSA (0.0.0.0/0) by the ABR.
Because the default route provides exit to external networks, candidates assume it is external.
Mistake
You can configure a stub area on any OSPF area, including area 0.
Correct
Area 0 (backbone) cannot be a stub area because it must carry all routes to prevent routing loops. Also, areas with virtual links cannot be stub.
Candidates think stub is a universal optimization without understanding backbone requirements.
Mistake
The 'no-summary' keyword must be configured on every router in a totally stubby area.
Correct
The 'no-summary' keyword is configured only on the ABR(s). Internal routers use the plain 'area stub' command.
Candidates assume all routers need the same configuration, but the filtering is done at the ABR.
Reveal each answer, then mark whether you got it right. Score 60%+ to unlock the next chapter.
No. The backbone area (area 0) cannot be a stub area because it must carry all routes to ensure loop-free inter-area routing. Stub areas rely on default routes, which would break the backbone's requirement to have complete routing information. Also, virtual-link transit areas cannot be stub.
'area stub' creates a standard stub area that blocks Type 5 LSAs (external routes) but allows Type 3 LSAs (inter-area routes). The ABR injects a default route. 'area stub no-summary' creates a totally stubby area that blocks Type 3, Type 4, and Type 5 LSAs, leaving only Type 1, Type 2, and the default Type 3 LSA. The 'no-summary' keyword is configured only on the ABR.
Yes. The ABR must also be configured with 'area <area-id> stub' to participate in the stub area. It is the ABR that generates the default route and filters LSAs. Without the command on the ABR, the ABR will not treat the area as stub and will not send the default route.
The neighbor adjacency will fail. OSPF routers include a stub flag in Hello packets. If one router has the flag set and the other does not, they will not form a neighbor relationship. The routers will be stuck in EXSTART or EXCHANGE state, and you will see errors in the log.
The default route metric (cost) influences which ABR a router prefers when multiple ABRs exist in the stub area. By default, the cost is 1. You can change it with 'area <area-id> default-cost <cost>'. A lower cost makes that ABR more attractive for default traffic. This is useful for load balancing or primary/backup ABR design.
No. Stub areas block Type 5 LSAs, so external routes from redistribution cannot enter the area. If you need to inject external routes, you must redistribute them at the ABR (which is in a non-stub area) and they will appear as Type 3 or Type 5 depending on the configuration, but they will not enter the stub area. Alternatively, use an NSSA (Not-So-Stubby Area) which allows Type 7 LSAs.
Both block Type 5 LSAs, but NSSA allows limited external routes via Type 7 LSAs, which are translated to Type 5 at the ABR. Stub areas do not allow any external routes. NSSA is used when you need to import a few external routes into an area that otherwise should be stub.
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