With a stub area, no external routes can be propagated into it. You could use a stub area, for example, for a branch office that has no connection other than to head office. In that case, you might make it totally stubby, that is, give it only a default route.
An NSSA is a bit like a stub as far as the internal OSPF topology is concerned, but it is allowed to connect to the outside world. In other words, it is allowed to have an ASBR border router. Imagine you had a branch office that had an external link, say, to the Internet. You could run that as an NSSA, but not as a stub.
Most popular implementations of Stub are spoke routers in Hub & Spoke topology. because the spoke has only 1 way to reach the outside world & thats through Hub. The spoke may be having LSDB for areas but no clue of reaching routes from extenal processes.So, it will have a default route to the ABR (hub in this case). The ABR will then route pkt. this is called a stub area where the routers in an area have a default route to ABR for reaching external routes.
Now suppose within a stub area theres an internal router which is redistributing routes in to ospf, thereby causing external routes in the stub. This defeats the definition of a stub as described above. So this are is now called as Not-so-stuby-area (NSSA). These external routes are propagate within area as Type7, but injcted out of this area to other ABR's as type 5.
Thanks.your post is very usefull to understand about Stub and NSSA areas.Now i have one more doubt.How do i create a default route in OSPF spoke routers in Stub area. is it same like creating default route in normal routers.tell me Syntax for creating default route in OSPF spoke routers in stub area
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