OSPF LSA Types Explained

Dear teachers, you are great. OSPF is now making sense to me to a larger extent! Let me throw you another question about LSDB. Is LSDB per unit area or per DR?

This Might Help To Clarify Network Types:

Broadcast multi−access (for example, Ethernet)A Broadcast network is one which allows broadcast
or multicast packets to be sent over the network and hence every device able to communicate directly
with every other device in the segment. A multi−access network is a network that connects more than
two devices. Refer to Initial Configurations for OSPF Over Broadcast Media for more information.

Point−to−Point (such as serial links with Point−to−Point and High−Level Data Link Control
(PPP/HDLC))point−to−point networks also allow broadcast or multicast packets to be sent over the
network, and these networks connect only two devices on the segment. Refer to Initial Configurations
for OSPF over a Point−to−Point Link for more information.

Non−Broadcast multi−access (NBMA), such as Frame RelayThese networks do not support
broadcasts or multicasts, but can connect more than devices and are multi−access in nature. Refer to
Initial Configurations for OSPF over Non−Broadcast Links for more information.

Point−to−Multipoint This is a collection of point−to−point links between various devices on a
segment. These networks also allow broadcast or multicast packets to be sent over the network. These
networks can represent the multi−access segment as multiple point−to−point links that connect all the
devices on the segment.

The LSDB is per area.

It was extremely helpful. I hope this would be my final question :). the question: Whats is the difference between LSDB per area and LSDB per OSPF process… Like there can be several areas in one process. So it again raises my curiosity about LSDB.

Hi Adil,

All routers have to agree on what the area looks like so that’s why we have a synchronized LSDB per area. If you have an area border router then your LSDB contains information from both areas. In that case, you have one OSPF process and your LSDB has information from two areas.


Hi rene,

You have said about link types above but didn’t give the complete picture. Could you please explain.

Let me explain them a bit more here:

  • Stub network link: this is a segment where we only have one OSPF router, a good example would be a loopback interface. An Ethernet interface where you don't have any neighbors also is a stub link. It doesn't have anything to do with stub areas.
  • Point to point network link: Used for serial connections (point-to-point) like PPP, HDLC or frame-relay point-to-point.
  • Transit network link: used for multi-access networks like Ethernet where you have more than one neighbor.
  • Virtual link network link: As the name implies, used for OSPF virtual links.


Hi Rene,
hope you are doing great.
some time back I had asked about the forwarding address field in the type 5 and type 7 LSA’s
you said you would add lesson for that. can you please let me know when yo add the same

Thanks Rene. Can you explain about different types of networks in Router LSA…? How to identify which type of network…? I mean either Stub,P2P,Transit…?
I want to understand how Link ID, Link Data are set in Router LSA…?


Hi Srini,

To understand this it’s best to check this lesson:

How to read the OSPF Database


Hi Rene,

Where are you advertising in the OSPF? How am I seeing the entry in the lsdb?

LSA Type 8 and 9 exists also in OSPFv3 (IPv6), mais be worth premising that this article deals with IPv4 only ?

Actually, its is clear that it is IPv4 only…

I love this lesson.
U r the best

Hi August. As far as I know, when you redistribute into ospf you can specify the external type (E1 or E2 - E2 is the default).

E2 will have a default cost of 20 and, when advertised to other routers inside ospf , the cost will keep in 20, so type E2 dose not consider the additional cost between other routers inside OSPF.

E1 will add the cost when advertised inside ospf. So, if you have 3 hops between the ASBR and other router with links running at 100 Mbps, you will have to add 3 to the cost, so you will end up with a cost of 23 at the far end.

Assuming someone hasn’t monkeyed around with OSPF’s reference bandwidth, you are exactly right.

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

Hi Rene,

Why we need LSA type 4 in OSPF? any specific reason we are using this.


Hello Ajay!

The LSA type 4 is used when you have multiple areas as well as an Autonomous System Border Router (ASBR).

First of all, an ASBR is a router that has at least one interface in an OSPF area (area 0 or otherwise) and at least one interface in another Autonomous System (i.e. a region of your network running another routing protocol, static routing or another autonomous OSPF implementation). The ASBR sends type 5 LSAs (external routes) to its OSPF area(s). The type 5 LSAs are flooded to all areas. However, they do not contain the necessary information to FIND the ASBR. The ABRs (which connect OSPF areas) which receive these type 5 LSAs will also send out a type 4 LSA to all other areas to inform all routers of the location of the ASBR. (This is quite well depicted in Rene’s original post with diagrams).

So, the type 4 LSA informs all other areas of the location (or the IP address) of the ASBR so they know the path that must be taken to reach the external routes advertised by the type 5 LSAs.

I hope this has been helpful!


Hi Rene,

In the below output, can you explain the output seen under “Net Link States”?

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         30          0x80000003 0x004CD9 2         31          0x80000002 0x0048E9 1

                Net Link States (Area 0)

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