Stub and Totally Stubby Area - Real Implementation

abdulwart

HI guys,

I have just learned the new terms of Stub and totally stubby plus not-so-stubby areas. Its wonderful concepts and I would like to know how those terms are frequently implemented in real life network.

Does anyone had the chance to do this implementation?

Again, i am working on Boson Simulator and the commands are there, however, doing the examples in CBTNuggests in the simulator, no response and the routing table is not minimized and no default route is added. Does anyone has the experince with Boson Simulator?

thanks

SysAdmin4066

I'll take a stab at this one...

First, I would suggest you start utilizing an actual lab, either real equipment or GNS/Dynamips, so that you can see these settings work in a "real" environment. Simulators are nice, but there are limitations as you're seeing. Here's the deal with the stub area types;

Basically, the two area stub types that are OSPF vendor neutral are Stub Area and Not So Stubby Area (NSSA).

Stub areas do not recieve any external route advertisments (type 5 LSAs), and routing inside the area is completely based on a default route. This reduces the size of the routing table.

NSSAs are a type of stub area that can import AS external routes and send them to other areas, but still cannot receive AS external routes from other areas. This is to ensure that external routes from outside the NSSA area are propogated internally (mainly redistribution), but the area is still considered a stub area for the AS. This is done by the use of type 7 Link State Advertisements (LSAs). Type 7s can be injected into NSSAs, but are then converted to type 5s at the border routers.

So here's an example; Your AS has 3 areas, 0, 1, and 2. Area 2 is classified as a stub area. This means that no external (type 5 LSAs) can be injected into this area. This is fine, so long as the area stays a stub and there is no possible reason external routes would need to be propogated. Now lets pretend the need arises to attach a new network running EIGRP and advertise this network into your AS. To do this, you'll need to redistribute routes and these redistributed routes will external OSPF advertisements. Well that's not going to happen if the OSPF area you're attaching the new network to is a stub area.

You'll need to convert it to a NSSA. What this does is allow the new ASBR in area 2 to advertise the external routes as type 7 into the NSSA. These denote an external route to NSSAs and are allowed to propogate through the NSSA. When the type 7 route hits the the ABR, the router converts the type 7 to a type 5 and it is advertised to the rest of the OSPF network.

The other stub area extensions you mentioned, NSSA TSA and TSA, are both Cisco proprietary. There are lots of Cisco docs on these areas.

Hopefully this is accurate and clear, anyone else wanna weigh in?