How to configure OSPF Stub Area

Previously I explained the different OSPF special area types called “stub areas”. In the next series of lessons I will show you the configuration of each OSPF stub type. Let’s start with the “normal” stub type. This is the topology that we will use:

ospf stub area

In the picture above we have 2 areas…area 0 and area 1. I’ll use this topology to demonstrate all the OSPF area types to you. Let me show you the configuration:

R1(config)#router ospf 1
R1(config-router)#network 192.168.12.0 0.0.0.255 area 0
R1(config-router)#redistribute connected subnets
R2(config)#router ospf 1
R2(config-router)#network 192.168.12.0 0.0.0.255 area 0
R2(config-router)#network 192.168.23.0 0.0.0.255 area 1
R3(config)#router ospf 1
R3(config-router)#network 192.168.23.0 0.0.0.255 area 1

I advertised all the interfaces in the correct OSPF areas with the exception of the loopback0 interface on R1. This interface is redistributed into OSPF so it becomes a LSA Type 5.

R3#show ip route ospf 
O IA 192.168.12.0/24 [110/2] via 192.168.23.2, 00:08:53, FastEthernet0/0
     1.0.0.0/24 is subnetted, 1 subnets
O E2    1.1.1.0 [110/20] via 192.168.23.2, 00:01:16, FastEthernet0/0

When we look at R3 you’ll see network 192.168.12.0 /24 as inter-area (LSA Type 3) and 1.1.1.0 /24 as external type 2 (LSA Type 5). Now let’s change area 1 in a stub area:

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Forum Replies

  1. It has to do with the design of the network in question. The benefits you see of an area becoming a stub (reduced LSDB size) comes at a cost, which is the loss of some routing information details. This translates into routers within a stub not having all the information necessary to make the best possible choices.

    For example, suppose you have an area (which is non-zero, of course), that has multiple exit points. Now imagine that at each of those exit points there are separate external routing domains (say, EIGRP or BGP, etc). If this area is a stub, Type-5

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  2. Thanks for making a video on this series. I am always thankful when I get to a video series as I read the book and website pages and just get worn out from studying 2-6 hours a day through the week. Not to mention on CCNP ROUTE I have taken to reading every single forum post as an added learning tool.

    Sometimes I just want to lean back in chair put on headset and listen to video as it allows me to relax a bit when tired towards end of day so uch thanks for those videos. Anyway great lesson!

    I am getting close to end of OSPF website lessons already finished

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  3. Hello sumu

    The NSSA area is an OSPF area where you know there are no other OSPF routers participating in OSPF beyond this interface, however, you know that there is an ASBR router found within that area. An ASBR is a router that connects to non-OSPF autonomous systems. No OSPF goes beyond this area, however, only other AS information should be relayed here. You can find out a more comprehensive explanation from Cisco at this Cisco documentation.

    I hope this has been helpful!

    Laz

  4. HI,
    I have a question on this point.
    Doesnt ABR just generate type3 LSA or type 4 to let us know of ASBR.
    Now in TSA, the only allowed LSA’s are 1 & 2. So how does ABR help ?
    if an ABR is configured with default originate always, my understanding is that route alwas appears as an O *E2 or Type 5 or External route in the downlink routers . But I am seeing it as O *IA in my setup, R1 – R2(ABR) – R3(TSA) , pls let me know why ?
    Addtionally in show ip ospf dbs cmd on TSA router , I see only LSA1 and Sumary Net link States (LSA3) being shown, should not LSA 3 be b

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  5. Hello Sahil

    An ABR will generate type 3 LSAs in order to inform other areas of the routes that are found in a particular area. Type 3 LSAs are sometimes known as a summary LSA, that summarises all networks within an area. A type 4 LSA is used to inform other areas of the existence of an ASBR.

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