OSPF Lab - Understanding all OSPF area types

 Set up the GNS3 lab as follows and configure the basic settings. 

Let's consider the R1 output, here we can see 10.1.2.1/32. It is not advertised with the correct subnet. Therefore we have to change the network type on the loopback interface to advertise it with the correct subnet. 

R1 configuration 

# interface loopback 1 

# ip ospf network point-to-point

Now we can see the 10.1.2.0 subnet is advertising correctly. 

Let's have a look at the R2 routing table.

R2 has directly connected networks and OSPF routes. Noticed R2 has no inter-area routes because R2 is in both areas, acting as ABR.

STUB AREA

Let's make area 2 a stub area. Here we need to configure both R2 and R3. Before doing the configuration, let's have a look at the R2 and R3 routing tables.

The R3 routing table is different. There are inter-area routes, O IA. 

After configuring the STUB area in R3 R3 has a default route pointing towards R2.

A STUB area has not received any external routes. It receives default routes and OSPF inter-area routes.

What advantage of having a default route rather than a more specific route? Router memory and processing are conserved because the router has fewer routes to contend with.

Why do all routers in the OSPF area need to know that the area is STUB? Routers need to know that an area is a stub for consistency. No routes generate LSA type 5. 

TOTALLY STUBBY AREA

We need to configure ABR only.

# area 2 stub no-summary

After configuring the totally stubby area, we can see a single default route from the backbone.

It will remove other routes and have only a default route.

Advantage TOTALLY STUBBY AREA instead of STUB area? Routers in the area only see intra-area routes and a default route. This can save a lot of router memory and processor time. However, if lost any route, it may lose to find the best optimal route.

Why need to configure only ABR other than all routers? ABR is the boundary and ABR passes LSA to others. So, it needs to filter out only LSA type 3 and the default route.

NOT SO STUBBY AREA (NSSA)

NSSA allows routes to be redistributed from an ASBR into that area with a specific LSA type, which gets converted to a normal external route at the ABR.

TOTALLY NSSA

Let's consider the R3 routing table.

R2(config)

# router ospf 1

# area 1 nssa no-summary

Now R3 will have only one default route, other than directly connected routes.

Where is useful to make NSSA? If want to allow an area to inject external routes into an OSPF domain while still retaining some of the stub characteristics of the area such as not accepting external routes that originated in other areas.

If you have doubts, please don't hesitate to comment below.