OSPF Areas

[iteration36]

I am having difficulty imagining what kind of network situations require what type of area. For example, Not So Stubby Areas and Totally Stubby Areas. I have read a great deal of documentation on these areas, but it is hard for me to visualize when to implement stub, totally stubby, and NSSA. Can someone with more experience help??

[vladrac-ccna]

Hello There,

There're some good links about OSPF desing considerations on cisco website.

http://www.cisco.com/warp/public/104/1.html

But I guess it depends mainly on what kind of devices you have (its cpu , memory) and what is the desing you intend to use. If the area you are setting is not a transit area, you can set it as stub with no problems.

Configuring a stub area reduces the topological database size inside an area and reduces the memory requirements of routers inside that area.

NSSA, it is only used if you are redistributing inside the stub area.

And totally stubby area is just a stub area where you only have default route coming in.

When to use it? Well, I guess whenever the admin effort justify it, or where you have low resource routers.

If you have a well design and hierachical network , I believe you wont use it.

but thats only my thoughts on it.

Vlad

[tony.costa]

The main factor you have to consider when you design an OSPF domain is topology.

A STUB area is often an area with few connections to the backbone (so with few ABRs connected to it), and you don't have the need to use the best path from any router in this area to an external network. This allows you to avoid the introduction of the external routes into the stub area (Type-5). Just a default route from the ABRs is enough and the internal metric for that default route is used for the path selection when sending packets to the external networks. However, an important condition for the definition of a stub area is that none of the routers inside the STUB area is redistributing routes into OSPF from another routing protocol.

A TOTALLY STUBBY area is mainly used when an area has just one ABR that connects it to the backbone. In this situation, you don't have the need to have all the inter-area routes (i.e. the routes related to networks belonging to other areas), since you don't need to optimize the path to all those networks: when you must send packets to networks outside the area, you always have to send them through the only ABR that connects to the Area 0. For this reason you just need one default route announces from the ABR to all the routers in Totally Stub area.

A normal NSSA is used when you don't have to optimize all the paths to the external routes advertized by the other areas (as in the STUB areas) but with an exception: one or more routers in this area are redistributing routes from another routing protocol.

In this case the routers in that area needs to reach the external routes directly through the ASBR but they always need to reach all other external routes through the ABRs which connect to the backbone, and so they can use a default network advertized by ABRs (as in STUB areas).

A Totally Stubby NSSA can be an area connected to the backbone through a single ABR (so for all the routes coming from other areas, internal or external, you just need the default route advertised by ABRs) but has one or more routers redistributing routes from another routing protocol.

I would say that a well design and hierarchical network MUST use these particular kinds of areas because this can avoid the insertion of useless routes inside the peripheral routers (often connected to STUB areas) and allows to choose less powerful routers for peripheral areas. I think these area types are the basis for the creation of well designed scalable networks.

I hope this can help.

Regards

Antonio

[iteration36]

Does anyone also know the difference between a Not so Stubby and a Totally Stubby NSSA? From the documentation, I see that the totally Stubby NSSA is Cisco proprietary. Is that the only difference? I am not really sure. I would also imagine that the bigger the enterprises/networks involved, the more likely that you would use a totally stubby or NSSA.

Also, has anyone ever been involved in the implementation of a Totally Stubby? What kinds of enterprises were involved?

THANKS

[tony.costa]

A Totally STubby NSSA doesn't want to know anything about all the routes coming from the other OSPF areas, so it receives just a default route from the ABR (like normal Totally Stubby Areas).

At the same time, the routers of this area need to use the external routes coming from ASBR (that is inside this area) which are not Type-5 routes (which are not allowed in any of this stubby stuff..) but are distributed as Type-7 routes. So Totally Stubby NSSA also behaves like Normal NSSA for this point of view (but remember just the default route coming from the ABR, not the Inter-Area Routes).

The applications?

When you use different routing protocols, sometimes you need to redistribute the routes between each other..

Imagine:

- OSPF in the core network

- RIP in some edge networks for some less powerful devices...

If a hierarchical network is designed, some areas can be connected to the backbone just through a single ABR (and this can justify the use of Totally Stubby areas).

At the same time, these areas can have routers redistributing between RIP, used at the edge, and OSPF. Therefore, these areas needs to be NSSAs (since the external routes must be propagated inside them).

So you must configure these areas as Totally Stubby Not-So-Stubby-Areas.

Regards