Like BGP neighbor adjacency states, OSPF has its own too. OSPF neighbor adjacency is not a bit straightforward. You might be expecting it should be in “FULL” state for the neighbor adjacency to be established, but you shouldn’t be assuming that it needs to stop at this state at all times.
OSPF IN DOWN STATE
Hello packets are very important parameters in establishing adjacency in any routing protocol not only in OSPF. Now, if no hello packets have been received from the neighbor and the dead timer interval has expired, OSPF is in DOWN state. The first OSPF neighbor state is “DOWN” state. It usually happens on Non-Broadcast MultiAccess (NBMA) networks and Non-Broadcast Point-to-Multipoint networks where neighbor is manually configured.
Stub Area is usually configured when there is only a single exit point on the network. It is like the dead-end of the OSPF network. This type of OSPF area only allows Inter-Area, Intra-Area and default route from ABRs. Thus, LSA Type 1, Type 2 and Type 3 are allowed to enter. It restricts LSA Type 5 from entering their zone. Recall the LSA Types on my previous post. LSA Type 5 is External LSA advertised by the ASBR. Routers configured as “stub” doesn’t care about external routes. Thus, you cannot see any O E1 or E2 on the routing table of the routers within the Stub area. Since it doesn’t bother to know LSA Type 5, there is no sense to know about LSA Type 4. As simple as it is, you cannot create virtual link in Stub Area and there is no ASBR as well.
It is very easy to understand Stub Area if you are going to configure and apply it in the laboratory. A few years ago, what I did is just memorize the concepts of OSPF Stub Area. But after being away from the Academy for so long, my own brain failed to store my memories about it. As we all know the brain is flexible in storing lots of lots information about what you’ve learned, experienced, your everyday life since birth, and so on. However, it is not a reliable storage most especially if you are not doing it every day (And it is not that quite memorable enough to be remembered).
One day, I was asked by an Erudite about the Stub Area.
“What is that again?” I asked my brain.
My brain whispered, “Uhm… I don’t know… I think there is nothing special on it.”
“What on bits-and-bytes!”
Analyzing the routing table and the result after configuration will help you understand the Stub area better than reading it a hundred times.