OSPF LSA types and summary routes

Dragon0x40 used Ask the Experts™
I am having a hard time understanding what the different LSA types do, which get blocked and where. Oh and why?

I have attached a screenshot of a Cisco explanation and when I read it I just get more confused.

Intra-area default route? How does that get generated and what does it contain?

Anyone like to take a stab at it or point me to a good website aritcle?
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OSPF can be confusing, I agree.  However, the stubby-area is quite simple to explain:  If an ABR sees that a non-backbone area is configured as "stubby" (this is something that must be spefified by the Netadmin for the entire stubby area), it will not forward any Type 5 LSAs it receives (from other areas) to that area.  Instead, the ASBR will create a Summary LSA (Type 3, I assume) for the route, and distribute that into the stubby area.  This will cause the OSPF process in the area routers to insert a default route ( into their routing tables, indicating the "shortest path" to the ABR that advertised this route.

I gained a lot of my OSPF knowledge from Cisco-specific courses.  Cisco training partners offer a 5-day course entitled "Implementing Cisco IP Routing (ROUTE)" that covers OSFP in much detail, among other routing protocols.


thanks Otto N,

Your answer is correct but any small mistake in documentation makes it difficult for me. For example you write that "the ASBR will create a Summary LSA (Type 3)" and I believe you mean the ABR will create the Summary LSA?

I suppose just having it explained 5 or 6 different ways is what it will take for me to finally understand it.

i usually re-read my answers before posting, but mistakes still happen...

Yes, it is the ABR that create the Summary LSA for the stubby area.

One thing you will need to keep in mind, is that LSA's are transmitted from neighbor to neighbor.  Even though the the same LSA originating on one end of an area reach the other end of the area, it is not directly transmitted (i.e. the same broadcast) from one router to all others.  LSA's are transmitted to directly attached neighbors, who then need to make a all on whether this LSA have to be forwarded to other neighbors.  In this way, an ABR that receives a Type-5 LSA will not forward it to neigbors discovered in a stubby area, and the routers in the stubby area will not be bothered by any type-5 LSA's.

Hope it helps.
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My angst about mistakes is not directed at you, it just adds a lot of time and frustration when trying to learn. I make a lot of mistakes myself!

My confusion is that I thought intra-area routes where LSA types 1 and 2 and the summary routes are at layers 3 and above.

Why recieve LSA type 1 and 2 if all the intra-area routes will be summarized in a Type 3 Summary?
Let's quickly review the main LSA types (1-5):

LSA Type 1 - Router LSA: Each router create it's own Type 1 LSA, which contain a list of all directly attached (working) IP prefixes.  Routers that receive this LSA, only forward it to other neighbors in the same area.

LSA Type 2 - Router LSA: Only a Designated Router (DR) on a transit network (i.e. a LAN segent) create a Type 2 LSA, which list the routers on the particular transit network.  Similar to Type 1 LSAs, routers that receive this LSA only forward it to other neighbors in the same area.

LSA Type 3 - Summary LSA (Inter-area):  This LSA is created by an ABR to advertise subnets learned via Type 1 and 2 LSAs to neighbors in other areas (since type1/2 LSA's does not cross area borders). By default, an ABR create a Type 3 LSA for each subnet learned, and does not summarise (group contiguous subnets together) by default.

LSA Type 4 - Summary LSA (ASBR list): A type 4 LSA is used to advertise to other areas the presence of an ASBR in a particular area.  It gets created by an ABR when it receives a type 1 LSA from an ASBR, and only list the router ID of the ASBR.

LSA Type 5 - External LSA: This LSA is created by the ASBR and list all subnets redistributed into the OSPF process.  This type is not limited to a particular area; any router that receives this type, will transmit it to all neighbor, regardless of area.

The point I'm trying to make, is that there is not much "hierarchy" that can be read into the LSA type number - Each type just have its own function, and it's own flooding scope: Type 1 & 2 is only flooded in the originating area, type 3 & 4 is flooded everywhere but the originating area, and type 5 is flooded everywhere, including the originating area.

So, to answer your question: Why recieve LSA type 1 and 2 if all the intra-area routes will be summarized in a Type 3 Summary? Because LSA type 1 & 2 will not leave the area, and the LSA type 3 created for these will not be flooded in the originating area.

I hope this answers your question, but let me know if it doesn't.  By the way, I was actually trying to appologise for confusing you in my previous post, but it seems I made a lousy job of that too... ;-).  I really have to learn to express myself better!


thanks Otto N,

That is the best description of OSPF LSAs that I have read!

I still have a few points that I am confused on but you cleared up 99% of them with this post.

If Type 3 LSA is not summarized by default then why is it named a Type 3 Summary? Why not Type 3 Inter-Area Prefixes or something like that? I have also seen documentation that states that Type 3 LSAs are summarized by default.

Is a Type 5 a summary LSA by default.

If a summary-address command at the ASBR or an area range command at the ABR is entered does that change the LSAs? Does the routing table on the ASBR or ABR get summarized or does the routers receiving the LSAs just get the summarized routes or both?

Does the default route advertised to a stub area change when it is converted to a nssa or a totally stubby area or does the default route injected into the stub stay the same and only thing that changed is that more LSAs are filtered and the routing table gets smaller.
Let me try to clear up the other questions you have:

I assume a Type 3 LSA is called a summary LSA since it can 'aggregate" a number of Type 1 & 2 LSA's into a single type 3.  For example, take two routers that are linked together by a LAN segment.  Both routers create a type 1 LSA each, which contain the LAN segment's IP prefix.  In addition, the DR for the LAN segment also create a type 2 LSA that list the other router as a neighbor.  Now, when an ABR receive these LSA's, it would only create one LSA for that segment, therefore "summarising" three LSA's (or a portion of three LSA's) into one.  This is my theory, at least, regarding the name of the Type 3 LSA.  Also, if you configure manual route summarisation, it affect the Type 3 LSA's.

When we talk about 'default' IP address summarisation, keep in mind that there is no default summarisation level OSPF uses.  What I mean, is this:  If an ABR receive, let's say, LSA Type 1 (or 2) for 4 subnets,,, and, it has no way to know that it can create one LSA for  Even if it did (since it receives all 4 in a single area), what happens if the route disappears?  Does it still advertise, or does it retract and advertise & instead?  So, by default, the designers of OSPF chose not to implement any IP address summarisation when creating a Type 3 LSA, except when explicitly configured for it.  Other routing protocols (like RIP and EIGRP) uses the class boundaries with auto-summarisasion: If it receives any subnet in the 10/8 range, it advertises to neighbors.  However, this can lead to confusion if you use addressing from different classfull networks, and this feature is usually turned off in production networks.

The same happens at the ASBR:  The ASBR would specify each specific subnet it learns from outside as a type 5 LSA, unless an explicit summary-address is configured on the ASBR.

The routing tables on the ABR/ASBR does not get modified by the 'area range' or 'summary-address' commands, but the number of LSA's get reduced, and the routers receiving the LSA will have a reduced routing table, as it received less LSA's

The default routes remain the same in stubby, totally stubby or NSSA areas:  The only change is which routes are allowed through the ABR into the area (and, in the case of NSSA, also out of the area).

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