OSPF Neighbor Adjacency States: From Down To Full

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.


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.

OSPF Neighbor Adjacency Requirements

Routing Protocols, unlike humans, can choose their neighbors. We, no matter what we do, cannot choose who’s going to rent or occupy the house near us. Yeah, we like to have handsome neighbors, neighbors with abs, educated persons, kind and thoughtful, clean and tidy, caring, and the most important one is someone with a stable job. However, in reality, it can be anyone. There is no such choice or whatever. Are we really looking for neighbors or fiance?

OSPF, like any other routing protocols, can agree or decline neighborship if their requirements are not met. Here are the adjacency requirements in order to form neighborship with OSPF:

1. same area ID
2. same subnet
3. same authentication type and authentication password
4. same hello and dead timers configuration
5. same area type

OSPF Area Types
OSPF Area Types


OSPF Packet Header Format

Have you ever wondered what is inside the OSPF packet header and how they communicate with other routers in the domain? The OSPF packet header varies depending upon the message type. Below is the common header format that the routers in the OSPF domain use to process packets.

Version NumberTypePacket length
Router ID
Area ID
ChecksumAuthentication Type
Message Body


The OSPF Virtual Link

The golden rule in designing an OSPF network is to have all the areas physically connected to the backbone area. No matter how many areas you want to have, they should be connected to the Area 0. This is the OSPF design and as what I’ve been saying since my previous posts, we cannot argue with how it is designed. The one who designed this have worked on this and tested this a lot of times. There are many trial and errors before this was officially brought to the public. I’ve been told that this is to prevent routing loops. Routers can exchange routes to all networks and if they are divided into areas, they cannot see others routes beyond their area and this would make them vulnerable to routing loops. I haven’t tried this kind of scenario on the lab that will show that routing loops occur when you don’t connect the areas to Area 0. One thing that I’ve tried is when you don’t connect an area directly to area 0, routes will not be learned by other routers in the other areas.

So here’s the Virtual Link. Virtual link is the solution to fix this broken OSPF design. You don’t need to physically interconnect an area but a logical connection in between a router connected to Area 0 and to a router connected to an Area not connected to Area 0. Confuse? Yeah, it is really hard when you just read it. Without digging deeper, as the name implies, it’s a virtual link. So let’s try to lab it!

OSPF Virtual Link
OSPF Virtual Link


OSPF Areas: Totally Not-So-Stubby Area (NSSA)

The last OSPF area that we will tackle is the Totally Not-So-Stubby Area (Totally NSSA)NSSA, like Stub area, has two flavours: one that is regular NSSA and the other one that is totally NSSA. The way it is configured is just the same with the Totally Stubby area where you are going to configure it in the ABR.

I will use the same lab on my previous post. I just changed the configuration on the Anopheles router and put the “no-summary” command.

Anopheles#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Anopheles(config)#router ospf 2
Anopheles(config-router)#area 20 nssa no-summ
Anopheles(config-router)#area 20 nssa no-summary

Aedes(config)#router ospf 3
Aedes(config-router)#no area 20 stub
Aedes(config-router)#area 20 nssa

It would be better to check the differences between NSSA and Totally NSSA using the before and after output of the NSSA configuration.

NSSA and Totally NSSA: show ip route
NSSA and Totally NSSA: show ip route


OSPF Areas: Not-So-Stubby Area (NSSA)

Do you know about Sergeant Stubby? He was the famous dog of high rank who has been commissioned to serve for infantry division of US in the first World War ever in the history. Though he was born a year or two before the Great War ends, it was said that he had served for 18 months as a war dog. At a young age, he had faced several battles on the field. Like any other men on the field, he got wounded a lot of times like foreleg injury due to German’s hand grenades.

Our topic for today is not about Sergeant Stubby but how a Stub network can be not so stubby. Isn’t it exciting?

OSPF Areas: The Totally Stubby Area

OSPF Totally Stub Area
OSPF Totally Stub Area

I just had a break after finishing the Stub Area lab. Yeah, I ate a lot today. One whole chicken chop with Naples’s aglio e olio, crunchy pork knuckle, bitter gourd salad, and banana shake. After a heavy meal, I became drowsy. Then, I sipped 2 cups of Cafe Latte. And, I think I am back. I’m good and I can still count from 1 to 10.

We are back with OSPF areas discussion and this time we will tackle about the Totally Stubby Area. You have to pronounce it right though if you know what I mean. (more…)

OSPF Areas: The OSPF Stub Area

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.

The OSPF Area Types

OSPF Area Types
OSPF Area Types

According to Cisco, OSPF area is a collection of logical OSPF networks (routers and links) that have the same area identification. Depending upon the design of your network, your routers can be part of a single area or multiple areas. Why do we have to complex things and put the router in different areas? Actually, there are many reasons, and one of the many reasons is that it reduces the size of OSPF database when OSPF network is divided into different areas. Think of it this way, dividing the routers into different areas, reduces the size of the database, reduces the frequency of SPF calculation and smaller routing table. Thus, fewer requirements on router memory and CPU. I am not saying that SPF calculation is exhausting the CPU and router memory but the sending and flooding of the new topology information does.

OSPF Link State Advertisement (LSA) Types

Link State Advertisement (LSA) is the building block of OSPF. You will hear this LSA over and over again when studying or working with OSPF. There are so many LSA Types that are very confusing at first sight. These LSA types are very important pieces to the OSPF LSDB puzzle most especially when configuring OSPF area types.

LSA Type 1Router LSA
LSA Type 2Network LSA
LSA Type 3Summary LSA
LSA Type 4ASBR Summary LSA
LSA Type 5External LSA
LSA Type 6Multicast LSA (MOSPF)
LSA Type 7NSSA External LSA
LSA Type 8 Link Local LSA for OSPFv3

LSA Type 1 or called Router LSA, as the name implies, represents the router. Each router within an OSPF area will flood LSA Type 1 and it stays within an area.

OSPF LSA Types: show ip ospf database
show ip ospf database