How to configure OSPF Summarization

To understand why an ABR installs the summary route to null 0, let’s imagine a really simple network with just two Routers: A and B with the following properties:
-Router A is in Area 0
-Router B is an ABR between Areas 0 and 1
-Router B has a default route that goes to Router A

Now, let’s say that Area 1 has the following networks:,, and You want to summarize Area 1 on Router B into Area 0. To do this, you would use an “area range” command with the summary address of Note: this summary address would include all of the following /24 networks:,,, and

Let’s imagine what would happen if Router B did NOT create the null route for that summary. Suppose Router A gets a packet destined for What will happen?

  1. The summary address from Router B includes the address of, so router A will send the packet to router B.
  2. Router B receives the packet, but recall from the scenario, that it doesn’t actually have the network in Area 1. Since it doesn’t have a specific route to that network, Router B will use its default route back to Router A. Router B sends the packet to Router A.
  3. Router A sends it to back to Router B because of the summary route.

This back and forth will continue until the TTL expires, and this is a routing loop.

So what does the summary route to null do for us on Router B? As you can imagine, it changes the behavior in step #2 above. When Router A sends the packet to Router B, Router B simply drops it rather than sending it back to A.

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hi Rene,
i have tried to understand summarization by myself, but only after your topic all became clear. Thank you for explanation and demonstration with practice.

19 posts were merged into an existing topic: How to configure OSPF Summarization

Can any body please explain me more about NULL 0 interface and AD for that route…

The null interface is typically used in summarization to prevent forwarding certain packets. Here’s an older reply by me that explains it:

Hello Sims,

I tried a different example from the one which are shown in the lesson, just to make sure that I understand OSPF summarization well enough. But unfortunately, I couldn’t make it work. Can you please review and let me know what’s done wrong? Thank you for your support.

R1-R3-R5 (Area 1)
R1-R2 (Area 0 - Gbps link* and reference bandwidth has been adapted to all routers to 1000)
R2-R4-R6 (Area 2 - but it’s not configured / used in this example)

R1 f1/0 and g0/0 interfaces have “ip ospf priority 2” to make the R1 the DR for all the segments.
R3 router-id: and lo if IP addresses are advertised in ospf with “ip ospf 1 area 1” command
R5 router-id: and lo if IP addresses are advertised in ospf with “ip ospf 1 area 1” command.

R1 ABR learns the routes via OSPF , marked as “O” in its routing table and same routes are LSA Type3 routes on R2 and shown as “O IA”. So at this stage, I return back to R1 to run the OSPF command "area 0 range " but still after this command the routes are not summarized in R2 and are shown as before.

Summary -routes for lo networks (R5 loopbacks) (R3 loopbacks)

Hi @tsirgogik

Which area range command did you use? On R1, this should work:

router ospf 1
area 0 range

You can make your summary a bit more specific btw, will cover -


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Hi Rene,
What about the external Route Summarization when redistribution happend on NSSA area . Its produce LSA-7 . The same command “summary-address” on ASBR ?? Thx


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Hello Mohammad

If you have a NSSA and you have a NSSA ASBR, then you can do route summarization for the redistribution into the NSSA area. It would still use the LSA type 7 to convey that information to the NSSA ABR which would then translate it to an LSA type 5 and propagate it to area 0 and beyond.

This is accomplished in the same manner with the summary-address command on the NSSA ASBR.

I hope this has been helpful!


Dear rene
kindly ,related with summerization option in ospf ,if advertice summery route only when have specific router fall in summery address , why i use no discard-route internal command under ospf process ?

Hello Saif

A summary will be advertised only if there is a subnet that is included within the summary. If that does not exist, then the summary route will not be installed, and no summary LSAs will be sent. Also, no Null0 route will be placed in the routing table. So in this case, the no discard-route internal command would be meaningless.

Now if you have a destination within the summary that has been configured, then a Null0 route will be installed in the routing table and summary LSAs will be sent. This is the normal operation.

Now if you want to remove the Null0 from the routing table, then in the second scenario you can do this with the no discard-route internal command as you mentioned.

An example of where this would be useful can be found in the following Cisco Support community post:

I hope this has been helpful!


hello lagapides

thanks for your feedback ,your clearfication is helpfull

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Hi Rene/Laz

Quick question! Why don’t we summarize within an area?

Hello sales2161

Simply put, we don’t summarize within an area because OSPF doesn’t support it. You can only summarize at an ABR between areas. Because OSPF is hierarchical and areas can be created, if an increased amount of summarization is really needed due to a large number of routing table entries, then the solution would be to create smaller areas. Because protocols like EIGRP are not hierarchical in nature, they are able to to improve scalability by summarizing at any EIGRP router in the topology.

I hope this has been helpful!


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The problem here is, he is trying to summarize area 1 but using area 0 in summarization command. Instead of area 0 range , you should use area 1 range

router ospf 1
area 1 range

Also the summary address (R3 loopbacks) won’t cover all loopbacks in R3.


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Hello NetworkLessons Team
Thanks a lot for clear explanation.

Help me please. i couldn’t choose right answers for question below:
Q: Which two areas does OSPF send a summary route by default?
Possible answers:

  1. NSSA
  2. Backbone
  3. Stub
  4. Normal
  5. totally stubby (not correct, because no type 3 summary LSA)

Is Normal area = Standard area or there is no “normal” area type?
All of Backbone, Standard, Stub and NSSA areas send LSA 3. Anybody can clarify?


Boris you are mixing a lot of things in this question what really should not be mixed.

As OSPF is link-state protocol you can summarize routes only on ABRs, but ABRs are not doing any route summarization for you by default, you have to configure route summarization manually.

However ABRs are summarizing for you LSA Type 1 and 2 into LSA Type 3 and this LSA Type 3 is after flooded to other area (that are the “O IA” routes). This is ok, because you dont need complete visibility to neighboring area, OSPF ABR is simply saying by this “If you need to send something to subnet in another area, send it to me and I manage the rest, because I have complete visibility there.”

Regarding to stub area types, there are really only 3 stub area types – Normal, Stub and NSSA. Backbone is not a stub area type. Backbone area is always Normal area, it cannot be Stub or NSSA.

Exactly opposite, there is no “Standard” area type. Its called Normal area.

Its not like areas are flooding something into other areas.
ABR routers are generatng and flooding LSA Type 3.
ABR routers that are interconnecting Backbone (Normal) area with other area, which can be Normal, pure Stub, pure NSSA or NSSA Stub, are flooding LSA Type 3 into them (the O IA routes).
It can also be Totally Stub and NSSA Totally Stub, when the area is Totally Stub or NSSA Totally Stub then LSA Type 3 is not flooded into them. Remember when you see the word “Totally” it means NO LSA Type 3 (in other words no “O IA” routes).

Now back to your question.

Question should be rephrased, because otherwise answers would not make sense.

Q: To which two areas does OSPF send a default route?

  1. NSSA – NO, pure NSSA area does not have a default route from ABR. This is sneaky.
  2. Backbone – NO
  3. Stub – YES, default route injected as LSA Type 3.
  4. Normal – NO
  5. Totally Stub – YES, default route injected as LSA Type 3. The only LSA Type 3 allowed in Totally Stub is the default route.
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Hello Boris

I agree with @fugazz that the question does not have the best wording, but this is a question that is found on some practice exams, so we have to take the question as it is. You may find something similar on an actual exam, so you will need to be prepared for bad wording.

Otherwise, it is indeed only the stub and totally stub areas that will have summarization flooded by default. For the totally stub area, the summarization does indeed only include the default route.

I hope this has been helpful!


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Thanks you very much

Thank you very much.