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Let’s check the what is Stub area, why we need Stub area and what are the different types of Stub area in OSPF.

Stub area is an Area in which external LSA’s are not allowed.

Let’s check why Stub Area is required and in what scenarios it is best suited.

1. Routers with less processing power, less bandwidth, where only within area OSPF routes are required.

2. Let’s take the below example, in which area 2  is non backbone area, ASBR is present area 0 i.e as we all know backbone area. Its might be possible ASBR has 100’s of external routes, so area 2 router’s needs to process this in their database as well, it would be better if area 2 had default route to ABR and then ABR will reach this ASBR.

This is achieved with the help of stub area.

In some Autonomous Systems, the majority of the link-state
        database may consist of AS-external-LSAs.  An OSPF AS-external-
        LSA is usually flooded throughout the entire AS.

Rules of Stub area:

  1. Stub area can be only that area with single exit point like area 2 in the above case, there is no further area connected to area 2.
  2. Backbone area i.e. Area 0 cannot become stub area because it has to carry all types of OSPF routes.
There are a couple of more restrictions on the use of stub areas.
        Virtual links cannot be configured through stub areas.  In
        addition, AS boundary routers cannot be placed internal to stub

The below buttons list all types of Stub area present in OSPF. You can click on the respective buttons to know more about each area, their restrictions, presence of default route originated by ABR and so on.  

STUB LSA 4,5 Blocked

TOTALLY STUB LSA 3,4,5 Blocked

NSSA LSA 4,5 Blocked, LSA7 allowed

Totally NSSA 3,4,5 Blocked, LSA7 allowed

I hope this has been informative to you, see you in next blog, stay tuned !!!!

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