Type 4 LSA

Unanswered Question
Jul 21st, 2007

Hi,

I have a doubt reg type 4 LSA in OSPF. Why do we need type 4 LSA to reach ASBR? We have ASBR address in type 5 LSA also. ASBR network is known via type 3 LSAs. Then why to send type4 LSA again?

Thanks,

Balajee

I have this problem too.
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mohammedmahmoud Sat, 07/21/2007 - 07:41

Hi,

ASBR Summary LSAs (Type 4) are identical to Network Summary LSAs (Type 3) except that the destination they advertise is an ASBR not a network.

HTH,

Mohammed Mahmoud.

srimural Sat, 07/21/2007 - 07:48

Hi balaji,

As posted by mohammed,

LSA 4 - Points to a destination IP , which is of a ASBR.

LSA 5 - is the external networks which the ASBR advertises, but with a next hop(Forwarding Address) only known to the ASBR. So inorder to reach those networks, the packet has to be forwarded to the ASBR. For this reason inorder to identify the ASBR Type 4 LSA is used.This is my understanding on the topic. Please correct me if I am wrong.

Thanks and Regards,

Sri

balajee Sun, 07/22/2007 - 22:23

Hi,

What you said is absolutely correct. But it did not answer my question. Let us consider the following diagram.

Area 1 - | R1 | - Area 0 - | R2 | - Area 2 - | R3 |

In the above diagram, I am redistributing routes in to area 1. R2 will generate type 4 lsa to reach R1. However, it will also generate a type 3 LSA through which R1 can be reached. My question here is - why do we need type 4 LSA once again while we have type 3 LSA to reach ASBR. ASBR address is known via type 5 LSA. Let us assume ASBR IP is 10.10.10.10. R2 propagtes type 3 LSA abt 10.10.10.0 network. Right!

Thanks,

Balajee

deepinder Sun, 07/22/2007 - 22:57

Hi Balajee,

Type 3 LSA (Network Summary)are generated by the Area Border router (ABR) and sent into a single area to advertise destinations outside that area but within OSPF domain.

Type 4 LSA (ASBR Summary) are also generated by ABR but advertise address of Autonomous System Border Router (ASBR) only.

Type 5 LSA (AS External) are generated by ASBR and advertise destinations external to OSPF domain or AS. These LSA are flooded throughout OSPF domain.

In short Type 3 will only advertise routes for destinations outside the area.

Type 4 will only advertise only address of ASBR.

Type 5 will only advertise routes that are external to OSPF (redistributed) reachable thru ASBR advertised by Type 4 LSA.

I hope this explanation makes ur understanting of OSPF LSA more clear.

If this info was of any help please rate it.

Thanks

Deepinder Singh Babbar

simonstoll Mon, 07/23/2007 - 05:15

Hi Balajee,

You are right that it can be sometimes a bit "double information", but consider that you can also filter out Type 3 LSA on ABR's but you still need to reach external Prefixes announced by ASBR, maby then the Type 4 LSA can be interesting. OK, thats whats in my mind if i try to figure out why not using the Type 3 LSA information, maby its complete rubish what I think :-)

Simon

balajee Tue, 07/24/2007 - 01:36

Yes, You got the point. May be you are right. What you said is one of the reasons, but there may be another causes also. I will try to investigate. Thanks Simon.

srimural Mon, 07/23/2007 - 05:32

Hi Balaji,

Got your question. The difference between the ASBR and ABR summary is that Type 3 is confined to the attached areas of the ABR. But Type4 and Type 5 is advertised throughout the OSPF Domain unless its a stub.

I hope it helps.Please correct me if i am wrong.

Thanks and Regards,

Srinath.M

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