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Stub Area For Beginners (ABR As Default Gateway For Internal Routers 3.1)

1. What is OSPF Stub Area?

Answer: In a stub area, OSPF internal routers don’t keep external destinations. They use ABR as default gateway to reach external destinations outside OSPF AS. That is, routers don’t have type 4, 5 LSA.

2. What is Totally Stub?

Answer: In this area, routers don’t even carry addresses from other OSPF areas. They use ABR as default gateway to reach destinations in other areas as well as external destinations outside OSPF AS. Routers in Totally Stub Area don’t keep type 3, 4, 5 LSA (except a type 3 default address 0.0.0.0)

3. Why Stub Area?

Answer: An OSPF internal router needs External LSA (type 5) and ASBR Summary LSA (type 4) to reach destinations in the outside world (non-OSPF addresses). However, when the number of external addresses grow to thousands, it’s getting difficult to maintain. To solve this problem, we can make the area stub: Internal routers don’t keep external addresses any more. They use ABR as the default gateway when want to access the outside world. This is very much the same way as hosts. They focus on running applications and rely on default gateway to access Internal.

4. Only ABR is configured as Totally Stub Area, not internal routers. Why is that?

Answer: Internal routers now don’t need to track OSPF network addresses from other areas. They rely on ABR as the default gateway. Therefore, they don’t need to keep Type 3 LSA anymore. They could delete Type 3 LSA when configured in an aream that is totally stub. But it takes several steps of manual operations. It is simpler to let the originator of Type 3 LSA – ABR – to cancel these LSA it announced earlier. ABR simply re-floods Type 3 LSA with age 3600.

This is more clear if viewing the topology in the companion simulation.

5. When configure ABR as a Stub Area router, it deletes type 4, not type 5 LSA. Why?

Answer: Type 4 is ASBR Summary LSA. It tells internal routers in how to reach ASBR when accessing external addresses. ABR creates and floods this LSA to internal routers. In a Stub Area, internal routers use ABR as default gateway to reach external addresses. They don’t need to know how to reach ASBR any more. Therefore, ABR does not need to keep Type 4 LSA.

But External LSA is different. If deleted, ABR won’t be able to forward packets to external destinations for internal routers. This is more clear if viewing the LSDB for LSA changes in the companion simulation.

6. When a host in a Stub Area ping an external destination, how is the ping packet being forwarded?

Answer: Here is the analysis of end-to-end forwarding path between a host and an external destination:

– The host forwards ping to its default gateway, which is an internal router.

– The router’s routing table does not have the external address, it forwards ping to its default gateway, ABR.

– ABR uses the external address to lookup its routing table, finds a match, and forwards it to ASBR.

– ASBR forwards ping to next hop in an alien AS.

– The next hop forwards ping to next hops until ping is delivered to the external address.

This article is the FAQ of a simulation from visualland labs. See OSPF simulation: Stub Area under External links listed below.

External links for network simulation:

1. OSPF Simulation: Stub Area (This article): http://pre.visualland.net/view.php?cid=760&protocol=OSPF&title=3.1%20Stub%20Area

2. More network protocol simulation: http://pre.visualland.net/

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