OSPF LSA Types Explained

Hello Ananth.

The information under the Net Link States (Area 0) show the DR(s) in the OSPF Area 0. The DR is the router with router ID and IP address For more information, take a look at this post: https://supportforums.cisco.com/document/133976/reading-and-understanding-ospf-database

I hope this has been helpful!


Rene , one question?

When you say in LSA type 5 , do you mean a route that is being redistribuited into RIP , and then into OSPF ? because Type 5 is External , and 4 mean when another domain as RIP sends traffic to our ASBR.

thanks in advance…!!!

You are right that a Type 5 LSA means an external route. This corresponds with any route learned by redistribution. It could be from another routing protocol (RIP, EIGRP, IS-IS, BGP, ODR, etc), or it could be from a redistributing static routes, connected routes, etc.

A Type 4 LSA does correspond with an ASBR, but not necessarily about traffic being sent to it. The Type 4 LSA tells OSPF routers who the ASBRs are and how to get to them. It doesn’t carry information external routes flowing from those ASBRs.

Hi Rene,

Thank you so much for you lessons.
I complete my CCNA in April and i want to start CCNP but first i want to gain experience.
I work in Network but more LAN so even if i think its better gain experience, i think i need to focus in study with book or in this site or i never will get the knowledge to get the CCNP.
What do you think is the better aproach to CCNP?
I agre with you to do first the ROUTE, my question is there are a minimum time of experience (Labor) to aim to CCNP or i can start right way?
I have 2 years of academic ,netacad and 10 months of working in area.
Thanks for yourpassion in teaching, i definitely will stick you networklessons in my path.

Hello Bruno.

It’s great to hear that you too have a passion for networking.

I can tell you from my experience with CCNA and CCNP that waiting to gain experience in a working environment to be able to more successfully complete my CCNP was of limited value. This is because the CCNP topics are very broad. No matter what kind of job you have in networking, you will never cover the broad range of topics that exist in CCNP. My suggestion to you is to start studying immediately, do as many labs as possible, and when you’re ready, go for it.

I hope this has been helpful for you!


Hi Rene,

It would be really helpful if you post lessons about isis protocol.
your way of explanation is really simple to understand.

Dear Rene,

I am confused why we need OSPF Area 0 and why should all the other Areas be connected with Area 0? Can you please explain?


The short answer: To prevent loops when dealing with multi-area topologies.

The longer answer:
The OSPF RFC 3509 has a section that talks about this. As I am sure you know, OSPF is a link-state protocol, but that is only true for OSPF behavior within an area. When OSPF starts to deal with multi-area typologies, OSPF actual behaves like a distance vector protocol. Distance vector protocols can have their own set of problems, one of which is the “count to infinity” problem. This is where two routers use each other to get to a particular destination, so the traffic will just bounce back and forth until the TTL expires.

Because OSPF behaves like a distance vector protocol between different areas, this means that OSPF also inherits this distance vector problem. To fix this problem, OSPF (and other link-state protocols like IS-IS), have a backbone rule. It requires all inter-area routes to flow through the backbone where a “split-horizon” like behavior will occur. This means that the backbone won’t advertise a route it learned from a particular area back into that same area. This stops the count to infinity loop.

Check out the RFC link I included for more details.

1 Like


I want to write CCNP Security 300-209 on September. As you know Cisco Book is not available for this exam. If you help me to get a good document to clear this exam.


If only there were such a document!

u rock!!

Per statement : “In this LSA we will find all the routers that are connected to the multi-access network, the DR and of course the prefix and subnet mask.”

Here Prefix means , IP address of DR which is connected in Switch - correct ?

Also in multi access network since we connect all routers via a switch: what kind of switches these are ? L2 or L3 ? Do we need to configure IP/VLANs anything on the Interface of switch that are connected to these routers ?

Hi Rene

when you say : We will find R1, R2 and the DR in the network LSA

So what exactly we find : is it Physical IP Address of R1/R2/DR OR loopback IP ?


Hi Abhishek,

Based on your question, I have made a small lab to show you what you can see in the database table (LSDB) when you have a multi-access network.

My example is based on 3 routers connected to a switch as following:

Interface G0/0 =
Loopback 0 =
Interface G0/0 =
Loopback 0 =
Interface G0/0 =
Loopback 0 =

After enabling OSPF on each of the routers and they form neighborship, R2 was elected as the DR and R3 as BDR as you can see below:

Neighbor ID     Pri   State           Dead Time   Address         Interface           1   FULL/DR         00:00:35     GigabitEthernet0/0           1   FULL/BDR        00:00:34     GigabitEthernet0/0

on R1, I have issued this command to see the OSPF LSDB

R1#show ip ospf database 
            OSPF Router with ID ( (Process ID 1)

                Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count         799         0x80000003 0x006eef 2         799         0x80000003 0x0082ce 2         770         0x80000003 0x0096ad 2

                Net Link States (Area 0)
Link ID         ADV Router      Age         Seq#       Checksum         770         0x80000002 0x007555

As you can see above, you have Router Link States listing all routers in the area, and the Net Link State listing the DR.

Hope this is more clear now for you.

Many Thanks…so this o/p of show ip ospf database showing us both router LSA and network LSA for this topology :slight_smile:

thx for clearing my doubt !

19 posts were merged into an existing topic: OSPF LSA Types Explained

Hi Rene ,

That was great explanation. Thanks a lot for this.

In the example of LSA1 , LSA2 , LSA3 types generation, I have setup the same lab in GNS3 but i cant see the DR abd BDR election.
and Hence below type 2 LSA’s are not visible in the in my ospf LSDB which is appearing in your database.

Net Link States (Area 0)

Link ID ADV Router Age Seq# Checksum 31 0x80000001 0x008F1F


COuld you please let me know if i am missing something here.


Hello Annapurna.

I’m assuming that the output you have above is from R1. Based on that, the router with IP address is the DR. Anything under the “Net Link States” section is the DR, so you do have a DR in your topology.

Can you be a little more detailed concerning the problem that you are facing?




“R3 is in another area than R1 so it needs to know where to find the ASBR. In the LSDB you can see the type 5 external LSA but also the type 4 summary ASBR LSA which is the address of R1. Because of this LSA, R3 knows how to reach the ASBR. This type 4 LSA is being generated by R2 which is the ABR”

Where id TYPE 4 LSA ?



The LSA-type 4 (Summary-LSA (ASBR)) is genearated by R2 in the topology explained .If yes why I Cannot see in R3’s ospf database