OSPF LSA Types Explained

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@lagapidis @ReneMolenaar
Q) if R3 is receiving type-3 from R2 already to reach ASBR then why do we require type-4 LSA?
Wouldn’t type-4 become redundant?

Hello Tejas

A Type 3 LSAs will only redistribute routes found within the attached OSPF area. So the Type 3 LSA that is being advertised by R2 to R3 that you see here is only advertising 1.1.1.0/24.

Type 5 LSAs will advertise the routes external to the OSPF area, and you can see this in the OSPF database, were 11.11.11.0/24 is being advertised.

Now, the advertising router is 1.1.1.1, and the type 5 LSA includes this information. But there is no guarantee that all other areas in an OSPF topology will have a route to this particular router, which is the ASBR. How does R3 know how to get to R1? The only way it can know this is through the Type 4 LSA that informs the whole OSPF topology of the existence of this ASBR and how to reach it.

I hope this has been helpful!

Laz

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Hello @lagapidis ,

I am still confused that if R2(ABR) is injecting 1.1.1.0/24 routes in area 1 then can’t the R3 just check that to reach 11.11.11.0/24 network we need to reach 1.1.1.1 & 1.1.1.1(or 1.1.1.0/24) is reachable via R2. So can’t R2 just route R3 towards ASBR(as R2 knows from LSA1 that where is ASBR).

R2 helps R3 to reach all networks in Area0 so can’t R2 just help R3 to reach ASBR?

Hello Tejas

Yes I understand your concern. It makes sense that if R3 learns about the 1.1.1.0/24 route and knows how to reach R1 to get to that route, then doesn’t it make sense to use an additional LSA to send that information out again. It sounds redundant.

Imagine however that there was no 1.1.1.0/24 network. R1 would send its information about the external route to 11.11.11.0/24 via a Type 1 LSA. R2, the ABR would send this information to R3 using a Type 5 LSA. That type 5 LSA, if you were to take a look at it in the OSPF database, would have a link-state ID of 11.11.11.0 and an advertising router of 1.1.1.1, as shown in the lesson. That means that R3 should send all traffic destined for 11.11.11.0/24 to 1.1.1.1. But where is 1.1.1.1? R3 will not have this destination in its routing table (unless it is explicitly advertised by R1, which by no means is certain).

In this case, you must have that extra piece of information that is found within a Type 4 advertisement, which tells R3 how to find the ASBR. If you were to look at that type 4 LSA in the OSFP database, you would see a link ID of 1.1.1.1, which is the ASBR router, and an advertising router of 2.2.2.2. So R3 will know that to get to the ASBR, its next hop address used should be that of R2.

An excellent and detailed description of all OSPF LSA types and how they appear in the OSPF database can be found in this Cisco documentation, along with a more detailed explanation of the importance and uses of Type-4 LSAs.

I hope this has been helpful!

Laz

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@lagapidis thank you soo much for a detailed explanation. This clears my doubt.

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Can you explain the LSA recursion process like , How LSA-3 recursion happens and how Lsa-5 recursion happens ??..Do u mean that LSA-5 is recursed to LSA-5.

What is the meaning of routing bit set , on which LSA these are set , what if there is no routing bit is set ??.

Why there is a LSA-4 when the ASBR is already adverting the LSA-5 , what is the use case of LSA-4 ?

What is the use of Forwarding address ?, what is the Non-zero and zero field in forwarding address ??

How LSA AGE is being maintained,Does that mean every hour the routers need to flood their LSDB once again ??

Hello Narad

Recursion is the process that an OSPF router takes to “solve” the shortest path algorithm. When a type 5 LSA is received, it must be “recursed” to the point where there is a type 1 LSA in the LSDB that points to the next hop router.

For example, take a look at this image:

R2 receives a type 5 LSA from R1 which is an intra-area ASBR, you will see the type 5 entry in the LSDB, but that entry will have a forward address of 0.0.0.0. This means that in order to reach the destination, the router must solve the shortest path to the ASBR. You will also see in the LSDB that there is a type 1 LSA that points to R1 which is the intra area ASBR. So R2 recurses the type 5 LSA to a type 1 LSA and solves the shortest path to reach that external prefix.

A similar process takes place with a type 3 LSA as well as a type 5 LSA that is received from an inter-area ASBR as well.

The routing bit is an internal implementation variable that is used only by Cisco. It is a bit that is set on an LSA only if the LSA should be considered during the SPF calculation. The bit is set only after the LSA has passed all necessary checks. This makes it more efficient so that these checks don’t have to be performed over and over. If the routing bit is set, the router knows that the LSA is eligible for processing. Note that this is a bit in the options field of the LSA and is not propagated from router to router.

Take a look at this post:

As stated in this Cisco documentation:

Data traffic for the advertised destination will be forwarded to this address. If the forwarding address is set to 0.0.0.0, data traffic will be forwarded instead to the advertisement’s originator.

You can find detailed information about this in the following lesson:

I hope this has been helpful!

Laz

Hi, I would like to ask, what is the difference between type 1 LSA & hello message. Both seems to share information about the router itself.

Hello Deep

First of all, let’s clarify the terminology. There are five message types that OSPF uses:

  • Hello
  • Database Description
  • Link state request
  • Link state update
  • Link state acknowledgment

LSAs of any type are not a type of OSPF message. However, all types of LSAs are contained within the LS Update OSPF messages.

Take a look at this lesson to learn more about these message types.

Now having said that, all OSPF messages are similar in structure. They all contain an OSPF header that contains information about the OSPF version number, a field that indicates the type of message, the area ID, and the source OSPF router of the message.

The rest of the sections of an OSPF packet depend on the type:

  • A hello packet will have a hello header, and an OSPF LLS data block
  • A database description will have an OSPF DB description header and an LLS data block
  • A link-state request will have only a Link State Request header
  • A link-state update will have only an LS update header (which contains the LSAs)
  • A link-state acknowledgment will have acknowledgments for specific LSAs that were received

You can take a detailed look at what these look like in the following Wireshark packet capture:

https://www.cloudshark.org/captures/111cb2076caa

I hope this has been helpful!

Laz

Thank you Laz, this is very clear and useful.

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My question is How do i minimize LSA type 4 translation in my network.

Translation process as LSA hits ABR
-If i have multiple ABrs how does my translation works?

Hello Prashant

Can you be a little more specific about what you want to achieve? A Type 4 LSA is necessary when you have an ASBR sharing routes from another routing domain. The Type 4 LSA is required to let other areas of OSPF know where to find the ASBR. One way to limit the number of Type 4 LSAs is to modify your network architecture to eliminate ASBRs, but this may not be possible in some cases. Please elaborate on what you are trying to achieve so we can help you further.

I hope this has been helpful!

Laz

Hello Prashant

Such a configuration works just fine. If you have two ABRs that connect two areas, say area 0 and area 10 together, then you will have two Type 3 Summary LSAs injected area 0. Both will inform routers in area 0 about routes that exist in area 10. Now if you want OSPF to prefer one ABR to the other, you can play around with OSPF costs to ensure that routing will prefer one ABR over another. More info about OSPF cost can be found at this lesson:

I hope this has been helpful!

Laz

Hello ,

I was going through ospf ,in this we have missed bits type & also after applying i cannot see OSPF database inspite of this you have only mentioned the show ip ospf routes ,so its hard to compare what lsa will be there when change in stub areas,
Please do reply me on anurag1991gautam@gmail.com

Hello Anurag

Thanks for your post!

When you say “bits type” are you referring to the OSPF options field that is found in hello packets, database description packets, and LSAs, correct? You can find out more about this particular feature in the following subsection of the related RFC 2328:

https://www.rfc-editor.org/rfc/rfc2328#appendix-A.2

The OSPF database can be viewed by using the command show ip ospf database. More information about this can be found at the following lesson:

I’m not sure if I answered your questions, but if you have any further questions feel free to ask them here.

I hope this has been helpful!

Laz

Perfect content, Thank you very much.

One question if you may, after reading some article, it seems to me that LSA 2 network type is kind of redundant, isn’t LSA 1 sufficient for routers to draw the whole picture of the network? why do we need LSA 2?

Thank you for the amazing content.

Hello Oussama

Indeed Type 1 LSAs would sufficiently inform OSPF routers of a full topology. However, Type 2 LSAs are used specifically by DRs. Remember that when you have a multiaccess topology (i.e. Ethernet) with three or more OSPF routers on the same subnet, elections take place to determine which routers on the network segment become the DR and BDR. This distinction requires that the LSAs sent by non-DRs (Type 1) and those sent by DRs (Type 2) differ. And they must differ because a Type 2 LSA contains information about all of the routers found on a particular network segment.

Theoretically, if we did away with DRs and BDRs, then we wouldn’t need Type 2 LSAs. But because we need them to make OSPF more efficient in especially larger topologies, we use the TYpe 2 LSA.

I hope this has been helpful!

Laz

In link type , there is link connected to stub network also there ?

Hello Sims

Hmm, I’m not quite sure what you’re asking, but I’ll do my best to share with you how LSAs work when you have a stub network.

In OSPF, a stub network is defined as a network with only a single connection to other networks. It means that the network only has one way in and one way out.

When Type 1 LSAs are stored within an OSPF router, they contain information about the connection to each of their OSPF neighbors. Within this information is found the type of link, and one of those types of links is the stub type. You can see that in the first table of the lesson.

If we take a look at a Wireshark capture of an OSPF update, we can see this information clearly. The following shows packet number 12 of this particular capture:

Notice that this OSPF update contains many different LSA types, one of which is an LSA Type 1. This type is expanded, and we can see the type of link that this particular LSA indicates, which is a stub. (See highlighted indicators). Does that make sense?

I hope this has been helpful!

Laz