OSPF Stub Areas

​

There are numerous stub areas. Stub areas are pretty easy to configure and easier to implement than EIGRP stub areas if you ask me. The point of stub areas is to reduce table sizes, look up times, LSA propagation, and allows for easy summarization at the ABR and ASBRs.

 

The following stub areas and their characteristics:

1. Stub Area (Industry Standard)

•       Type 5 LSAs (redistributed routes into OSPF) are rejected

•       Only one entry and exit point

•        Internal routers in a stub exit require a default route provided by the stub ABR.  

•       Allows O IA routes, rejects E1/E2 routes

•        No ASBRs allowed in the basic stub area

•        Area 0 cannot be any stub area

•        Cannot be a virtual-link transit area

•        Internal routers must be configured as stub as well to form adjacency

•        Stub areas reduce memory and table size

 

2. Totally Stubby Area (TSA; Cisco Proprietary)

• Similar to the plain stub area

• Configuration differs by applying the “no-summary” keyword on the ABR, and that’s it. The “no-summary” keyword prevents type 3 and 4. The “stub” command prevents type 5 LSAs

• Rejects O IA routes, rejects E1/E2 routes. (blocks type 3, 4, 5 LSAs)

• Only routes permitted in the area is a default O IA route and O routes that are local to the area

• Default route cost is provided with a cost of 1 unless specified

• Configuring an area as TSA reduces the route table and topology size even further than the traditional stub area.

• With a loss of route-detail due to configuring the area as a TSA, a non-optimal route to a remote destination may be chosen.

 

3.  Not-So-Stubby Area (NSSA; Industry Standard)

• Used when redistributing an external protocol or static routes into OSPF. An example may be joining one part of the internal network that is EIGRP with a new addition that is OSPF

• We may use this if we don’t have Cisco equipment to run the totally stubby version of NSSA.

• NSSA area requires an ASBR due to redistribution of external routes

• The ASBR generates a type 7 LSA indicating redistributed routes

• The ABR of the NSSA translates this type 7 LSA into type 5 for the rest of the OSPF domain

• The ABR shares a type 7 default route when configured with the “default-information originate” parameter. ASBR requires only the “nssa” parameter

• The ASBR generates a default only when learned through a non-OSPF protocol

• Similar to what a “stub area” is for basic OSPF stub routing

• Useful for injecting external routes into OSPF while not accepting external routes originating from other areas (E1/E2, N1/N2)

 

4. Totally Stubby Area Not-So-Stubby Area (TSA NSSA; Cisco Proprietary)

• An alternative to the NSSA

• Combines TSA behavior with NSSA

• Works when you have the Cisco equipment needed here

• ASBR injects external routes while only accepting a default O IA from the ABR

• Again, the ABR translates the type 7 LSA into type 5 for the rest of the OSPF domain. Other areas see the external routes as E1 or E2 routes.

• The ABR itself sees these external routes as N1 or N2 routes

• ASBR may only generate a default when it has one in its table from a non-OSPF protocol

• ABR may generate a default without one in its table

• On the ABR, issue the “no-summary” keyword to activate the TSA NSSA function

 

With the following topology, we can show how these features work. Starting on R4 and R5, we can implement a stub area, and then a totally stub area:

​

​

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Initially, without any stub configuration, R4 has ALL routes explicitly in its table as shown:

​

​

​

​

​

​

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

A stub feature is limiting the size of the route table. By issuing #show ip route summary, we can see the size of the table is about 6,000 bytes (very small)

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

Now: to configure the stub. Stub configuration is bidirectional so it must be consistent on the other end. Configuring it like so causes it to lose neighborship with R2. Simply under the process “#area 245 stub”:

​

​

​

​

​

​

​

​

​

​

​

​

​

 

To bring up adjacency, we have to set it the exact same command on R2. Once we do, adjacency is reestablished and we can see the results of the command in the route-table. As previously stated, the external E2 route is now rejected while IA routes are still permitted. We are designated by a default route by the ABR:

 

​

​

​

​

​

​

​

​

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Furthermore, the table to begin with was not very big, but we can see the size decrease by once again issuing #show ip route summary. Through summarization, we have cut off about 700 bytes. Again, not much, but the point stands and the benefits are evident:

 

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

 

Totally Stubby Area

As mentioned, very similar to the previous configuration. On R2, because it is the ABR, we use the command under the OSPF process “#area 245 no-summary”. Instead of blocking Type 5 LSA, the no-summary parameter also rejects types 3 and 4 LSA. Look at the size of the R4 route table now:

 

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

After issuing the “no-summary” parameter, R4 is given ONLY the type 3 default route. The only exception for type 3 LSA. Also, look at the size of the table now in bytes: it has been cut by more than half. An effective way of reducing route table size and bytes.

​

​

​

​

​

​

​

​

​

​

​

​

 

 

 

Not-So-Stubby Area

Switching sides of the topology since this requires redistribution, the focus will be on R3 and R7. We can assume that EIGRP was a different internal network running EIGRP and we wanted to merge it with OSPF. However, we don’t want to propagate ALL these external routes throughout the OSPF domain. So, we can use an NSSA.

Right now the route tables on R3 and R7 look like:

​

​

​

​

​

​

​

​

​

​

​

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

We can see here that R3 is receiving the external route as E2. Therefore, based on that alone, NSSA is not in effect.

Likewise, R7 is still receiving all the OSPF routes when it shouldn’t if NSSA was in effect:

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

 

Configure the two routers accordingly:

R3(config-router)#area 367 nssa default-information-originate
R7(config-router)#area 367 nssa

After we configure R3 and R7 as NSSA, we should see several things:

1. R3 sees the external EIGRP route of 192.168.7.0 as N2 instead of E2
2. R7 receives a static default N2 from R3
3. Other routers in the OSPF domain not in a stub area see the 192.168.7.0 route as E2 due to NSSA ABR translation from type 7 to 5
4. R7, because it receives a static default N2 from R3, it will no longer receive external routes from other OSPF routers in the domain. The NSSA rejects type 5 LSA (E1/E2)

 

So, after the initial configuration, R3 does in fact see the external route from R7 as N2 instead of E2.

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

 

 

Because the NSSA ABR translates type 7 into type 5, we can verify this on another router in the domain, such as R1:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

R1, as expected, receives E2 routes from R3, and not N2, because of translation.

We can also verify that R7 does not receive external routes from other routers in the OSPF domain. The ABR acts like a buffer and either translates routes external routes or rejects them. It rejects type 5 and translates type 7.

Lets take a loopback on R1 and redistribute it as an external route to see if this actually happens:

R1(config)#int lo200
R1(config-if)#ip address 200.200.200.200 255.255.255.0
R1(config-if)#router ospf 1
R1(config-router)#redistribute connected subnets

R3 receives the route as it should, but R7 does not because R3 denies the LSA:

 

R7 does not receive the 200.200.200.0 route, but we can still reached while sourced from the EIGRP network it thanks to the N2 default shared by R3:

 

 

​

​

​

​

​

​

​

​

​

​

​

​

​

​

 

Totally Stubby Area Not-So-Stubby Area

Similar to NSSA but it reduces the R7 route table even further. NSSA is like the first stub configuration shown earlier. TSA NSSA is like the TSA stub configuration shown second. They do the exact same thing but involve an ASBR.

The only noticeable change is on R7 – ALL O IA routes are rejected, as well as external routes, and we only see a default O IA, O routes, and the EIGRP route. The table is much smaller and easier to look at.

And we can still reach the external route that R1 is redistributing. This could essentially be the “Internet.”

 

 

​

​

​

​

​

​

​

​

​

​

​

​

Conclusion:

The benefits of OSPF stubs are obvious: smaller tables visually and in terms of bytes, reduced LSAs, and interoperability between routers that may be other vendors.We can also make effective use of summarization at the ABR and ASBR – covered in another post.