How to configure OSPF Summarization

This topic is to discuss the following lesson:

Thanks a ton to clear summarisation of LSA type 3 and LSA type 5 so easily…The website has been helping a lot in my learning in CCNP R&S.

I am indebted to you…

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Hi Rene,
I want to ask for summarisation on ospf.
If we do a summary and one router behind router Donna down, how the other routers know that one router is down and change their LSDB so do not send ip packet to it?
Whether OSPF not have or work lika a Split-horizon on RIP ??

Ael Irsal

Hi Ael,

Split Horizon is only for distance vector based routing protocols like RIP and EIGRP. OSPF doesn’t need this as routers within the area know what the topology looks like…the LSDB is synchronized.

When a router goes down, the neighbor adjacency will be removed and as a result LSAs will be removed and SPF will be re-run.

I have spend thousand of dollars training on my CCNA and CCNP and different instructors teaching but the way you explain with small lab and verification has help me to learn and clarify doubts I had.
It’s nice you keep this chapters short. You were born to teach.

Hi Alfredo,

Thanks for your kind words, I’m really glad to hear that my material is so helpful to you!


I agree the same, it’s very clear and useful.


I cannot get this to work in a three router topology R1R2R3. R1 & R2 are in Area 0. R2 & R3 are in Area 23.
On R1 I’m seeing type 3 and type 5 routes from R3 (172 & 10 networks). However, on r2 (the ABR) when I put the following config under the OSPF process:

 area 0 range

I still see the full type 3 & 5 routes on R1. For some reason they’re not being summerized.

Hi Bruce,

On R2, what O IA and O E2 entries do you have in the routing table?

The area range command is for the O IA routes and the summary-address command is for the O E2 routes.



I figured this out. Once I moved the ‘summary-address’ from R2 to R3 (the owner of the 172.16 routes) I was able to see summarized routes on R1. So, in summary (pun intended), when summarizing routes, summarization needs to be done on the router who owns those networks.

As far as the ‘area 0 range’ command; changing the config to ’ area 23 range’ did the job. So the ‘area X range’ command needs to face the area sourcing those networks (area 23 in my case). Not the area we’re advertising the range (area 0 in my case). Whew.

Thanks for your help


Hi Bruce,

Good job. the summary-address has to be configured on the ASBR while the area range command is for the ABR. Using the correct source area is easy to forget :slight_smile:


HI Rene

Can you pl explain below mentioned statement by giving some example -

A summary route will have the cost of the subnet with the lowest cost that falls within the summary range.

Hi Lokesh,

Let’s say you have these routes: - cost 10 - cost 20 - cost 5 - cost 30

When you create a summary route that matches these 4 routes then we will have to pick a cost. By default, it will use the lowest cost for the summary route…5 in this case.


rene, does this apply on Inter-area and External? or only INter-area? or only External? thanks!

Hi John,

the “area range” command is for inter-area prefixes, the “summary-address” command is for external prefixes.


what im talking about is picking the lowest cost. sorry i didnt clear my question.

Ah ok…I think this also applies to external routes but I’m not sure how it will behave with O E1 and O E2 routes (or a mix of those two). That would be nice to lab :slight_smile:

Hi Rene,
Thanks for your nice explanation on this topic. I have one query here. All the IA routes shows the metric as 2 and the E2 routes shows metric as 20, but physical topology is exactly the same. what we are really achieves here with this change in metric?


Hi Ashok,

In this topology, it doesn’t matter. It’s just that the default cost of E2 routes is 20.


Donna(config)#router ospf 1
Donna(config-router)#network area 0
Donna(config-router)#network area 1

Why is the ( wild card mask used? If they are /24 networks why not use (