OSPF LSA Types Explained

(Ananth Maruti M) #113

Can u explain for below type-3 LSA how the link-id and adv router is determined

Summary Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
3.3.3.3         2.2.2.2         17          0x80000001 0x00D650
192.168.23.0    2.2.2.2         66          0x80000001 0x00A70C
(sims) #114

Hi,

What happens is that R1 will flip a bit in the router LSA to identify itself as an ASBR. When R2 who is an ABR receives this router LSA it will create a type 4 summary ASBR LSA and flood it into area 0. This LSA will also be flooded in all other areas and is required so all OSPF routers know where to find the ASBR.

Can you explain " flip a bit in the router LSA"

Thanks

(Lazaros Agapides) #115

Hello sims

When sending LSAs, if the sender is an ASBR, then there is a specific bit in the LSA headers that is flipped in order to indicate that it is indeed an ASBR that is sending the LSA. Specifically, if it is 1 it is changed to 0 and if it is 0 it is changed to 1. I have been unable to find the specific field that is changed, however @ReneMolenaar may be able to more sufficiently answer your question.

I hope this has been helpful!

Laz

(Robert G) #116

Hi All,

I am reading the following when talking about R1 redistributing rip routes.

“What happens is that R1 will flip a bit in the router LSA to identify itself as an ASBR”.

Is this bit flipped in all the router LSAs (type 1) from R1or is this bit flipped only for a specific router LSA from R1?

(Lazaros Agapides) #117

Hello Robert

R1 will flip a bit in the LSA Type 1 because it is an ASBR. The flipping of the bit occurs only for Type 1 LSAs.

I hope this has been helpful!

Laz

(Faraz S) #118

Hello Network lessons team,

thank you so much for this good explanation, can you please provide me some other topologies for practice. please

(Lazaros Agapides) #119

Hello Faraz

Take a look at Rene’s GNS3Vault page that has many lab scenarios for many networking technologies. The link below shows those for OSPF and include topologies for examining LSA types as well:

I hope this has been helpful!

Laz

(bahri a) #120

R2 will translate this type 7 into a type 5 and flood it into the other areas.
is it true.

(Lazaros Agapides) #121

Hello Bahri

Yes this is true. R2 is an ABR and it will receive the type 7 LSA from the NSSA area and will incorporate its information into its OSPF database. It will then send out a type 5 LSA into Area 0 with the information learned from the type 7 LSA.

I hope this has been helpful!

Laz

(Mike C) #122

Hello. I was hoping an elaboration can be provided as to why Type 2 LSAs are needed. I went to the Cisco Press book and it states that OSPF has to see things as Node -> Link -> Node so it uses LSA Type 2 to model this in situations when there are more than two routers on a link. My problem is with all the verbose jargon in the book. I was hoping someone a lot more knowledgeable than I am could provide a more clear elaboration as to what the Type 2 LSAs do. Thank you.

(Lazaros Agapides) #123

Hello Mike

Type 2 LSAs are sent by DRs to all DROTHER (non-DR and non-BDR) routers in a multiaccess network. A multiaccess network is one where you can have multiple routers connected to the same network segment such as in the following topology:

Such a topology requires the election of a DR and a BDR. A DR will accept all Type 1 LSAs from other routers within the specific broadcast domain (not the area mind you but the broadcast domain). It will then compile all the information found in all of those Type 1 LSAs and create a Type 2 LSA. It will then send this out to all of the DROTHER routers in the broadcast domain.

The very purpose of the Type 2 LSA is to be used solely by the DR (and BDR) in a multi-access network environment. Even if you only have two routers connected with each other via an Ethernet connection, they will still have a DR/BDR election and use both Type 1 and Type 2 LSAs just because the Ethernet technology is inherently multiaccess. Conversely, Type 2 LSAs will not be found in any point to point technologies such as serial connections between OSPF routers.

More info about DRs and BDRs can be found here:

I hope this has been helpful!

Laz

(Mike C) #124

Hello. I was hoping an elaboration can be provided as to why Type 2 LSAs are needed. I went to the Cisco Press book and it states that OSPF has to see things as Node -> Link -> Node so it uses LSA Type 2 to model this in situations when there are more than two routers on a link. My problem is with all the verbose jargon in the book. I was hoping someone a lot more knowledgeable than I am could provide a more clear elaboration as to what the Type 2 LSAs do. Thank you.

(Lazaros Agapides) #125

Hello Mike

Please take a look at the post above:

I hope this has been helpful.

Laz

(Mike C) #126

Still can’t say that I completely get Type 2 LSAs…seems like Type 1s could do the trick alone.

(Lazaros Agapides) #127

Hello Mike

Let me try again. Here are a few fundamental differences between Type 1 and 2 LSAs

  1. Type 1 LSAs by definition are sent from an OSPF router to ALL of its neighbors while Type 2 LSAs are sent only from DRs to all routers in their network segment.
  2. Type 1 LSAs contain information about routes that a single OSPF router knows about while Type 2 LSAs contain information regarding all OSPF routers in the LAN segment. This allows the OSPF algorithm to use a single Type 2 LSA to reference all routers in a segment. Type 1 LSAs cannot do this.

Now you are correct when you say that Type 1 LSAs are sufficient to do the job. However, there is a great gain in efficiency if you use Type 2 LSAs. The benefit is experienced when you have many, say, 10 OSPF routers on the same network segment, and you have no DR or BDR, then you would require N*(N-1)/2 adjacencies. That’s 10*(10-1)/2 = 45 adjacencies. That will require 45 exchanges of Type 1 LSAs in total just for that network segment to converge. This results in a lot of CPU and memory usage as well as bandwidth. Using Type 2, you only require 9 Type 1 LSAs sent from all routers to the DR plus 9 more Type 2 LSAs to be sent from the DR to all the other routers. This is a vast improvement.

I hope this has been helpful!

Laz

(Mike C) #128

Thank you very much!

1 Like
(sims) #129

When talking about the lsa 4 , what if we redistribute connected networks ?

(Lazaros Agapides) #130

Hello Sims

A type 4 LSA is one that is created when an ASBR redistributes its subnets that participate in a particular routing protocol, into OSPF. For example, if R1 has 10.10.10.0/24 on interface Gi0/1 which is participating in RIP, and it is redistributed into OSPF, then this would be contained within a type 4 LSA.

Conversely, if you redistributed directly connected networks that are not participating in OSPF, then you are using a type 5 LSA to convey this information to other OSPF routers.

I hope this has been helpful!

Laz

(Lukasz W) #131

Hi Rene,

I think I found small typo in section “Let me summarize the LSA types for you:”
“Type 7 – External LSA: also known as not-so-stubby-area (NSSA) LSA: As you can see area 1 is a NSSA (not-so-stubby-area) which doesn’t allow external LSAs (type 5). To overcome this issue we are generating type 7 LSAs instead.”

On drawing NSSA its area 2 :wink:

Best regards,
Lukas

1 Like
(Rene Molenaar) #132

Hi Lukas,

Late reply here. I just fixed this, thanks for sharing :slight_smile:

Rene