OSPF Stub Areas

OSPF Scalability is done by minimizing the following:
- Reachability Information
- Flooding Domains
Areas
- Do not hide reachability information, but hide graph from other areas.
- SPF only runs inside area (intra area)
Reachability summarization
- Reduces number of prefixes.

Per-LSA Summarization
- Removes all IA routes, replaces with teh shortest match possible, a default route.
  - This is how Stub Areas work.
Filtering
- Enforced at ABR
- ABRs control which LSAs enter neighboring areas.
  - Types3, 4, and 5 are filtered depending on the Stub area type.
  - All routers in the area must agree on specific stub flag.

STUB AREA TYPES:

Stub Area:
- Stops external routes
  - LSA Type 5
Totally Stubby Area:
- Stops inter-area and external routes
Not-So-Stubby Area:
- Stops external but allows local redistribution.
Not-So-Totally-Stubby Area:
- Stops inter-area and external but allows local redistribution.

Stub Logic:
- Stub router/area knows how to reach ABR.
- ABR knows how to reach ASBRs.
- ASBRs know where external routes are.
  - ALL THAT IS NEEDED IS DEFAULT TO ABR.
Technical results:
- ABR removes LSAs 4 and 5.
- ABR originates default route into stub area.

Stub Configuration:

Above there’s three routers:
R2 – Area 0, advertising Loopbacks to R3
R3 – Area 0 to R2, Area 1 to R4

R4 is receiving the following routes via OSPF, including the E2 externals being redistributed from outside the AS to R2.

To change area 1 to a Stub area, the below commands will be run on R3 and R4:

After the commands are entered, the adjacency between R3 and R4 goes down, then back up. The routing table on R4 now shows below:

There is a default route and no longer any external E2 routes from the redistribution. This is because Stub will remove only the LSA 4 & LSA5. LSA 3 (IA) is still showing up in the routing table.

Totally Stubby Logic:

Know how to reach ABR.
ABR knows how to reach other areas and ASBRs.
ASBRs know how to reach Externals.
If default to ABR, no need to External routes or inter-area.

Totally Stubby results:

Removes LSAs 3, 4 and 5.
Adds default from ABR into area.

Totally Stubby Configuration:

Continuing on with the three router example (4 if counting routes that are coming from redistribution on R2, the area 1 ABR needs an addition to its stub command. no-summary

Now on R4’s routing table, all that should be available via OSPF is a default route from the ABR (R3).

Not-So-Stubby Area Logic:

Filters like Stub area but allows local redistribution.
- Local redistribution is LSA type 7, which gets turned into LSA type 5 by ABR for advertising into rest of OSPF domain.
No default route provided from ABR into NSSA.

NSSA Configuration:

R4 is still configured as a Totally Stub, so it’s only receiving a default route from the OSPF domain. In addition there is now R1 that is an EIGRP neighbor advertising the route 96.76.43.136.0/29 to R4.

When trying to allow redistribution from EIGRP into the Totally Stub OSPF domain, an error occurs saying it cannot be done. In order to do this the command ‘area 1 nssa’ needs to be added on both R3 and R4.

The routing table now on R4 receives no default route from the OSPF ABR and no external routes from any other area except for its own.

the routing table now on R3 (ABR) receives N2 routes showing that locally redistributed route from R4.

Not-So-Totally-Stubby-Area Logic:

Blocks inter-area and External, allows local redistribution.
Combines NSSA with Totally Stubby.

Result:
- Redistributing router generates LSA 7
- ABR Changes to LSA 5 when advertising into rest of OSPF domain.
- ABR Removes LSA 3, 4, and 5.
- ABR Originates default route.

Not-So-Totally-Stubby-Area configuration:

Same topology above, the configuration needs to be changed now on R3. Currently R3 and R4 running area 1 are setup as a NSSA, so command ‘area 1 nssa’ is added under each router ospf process. Below is the only configuration change that’s needed.