OSPF: Area range vs Summary address

It seems like an easy one! I mean what could be so hard about area range and summary address command? You will be surprised how many people tend to forget this things or to apply them when not necessary or even worst where is their place to be added. It’s basic knowledge, but sometimes exactly this basic knowledge give us headache.

In the following line, I will try to explain briefly the difference between Area range and Summary address command. First let’s have a look at the following topology:

We have mixed network domain with EIGRP and OSPF. From documentation we know that along other routers in OSPF domain we have 2 important routers:
ASBR (Autonomous System Boundary Router)One device at the edge of OSPF domain, that receive routes from another non-OSPF domain (e.g. RIP, EIGRP…)
ABR (Area Border Router) –
One device inside the OSPF domain that assure Inter-area communication (e.g. area 0 and area 1)

You will find situation when only one physical device will be ASBR and ABR router. The functionality and rules remain the same, just that instead of having 2 device you have only one that connect to another non-OSPF domain and in the same time to multiple OSPF areas.

Why we would use this 2 commands? Well one simple explanation is that we want to summarize advertised subnets to reduce the total number of routes present in the routing table. Less routes means less overhead and load for a router.

Let’s have a look to the routers presented in the above topology. The dynamic IP routing protocols (EIGRP and OSPF) are already configured and functional. For the IP network clouds, I have used Loopback interfaces.

R1 - Interfaces and EIGRP config
On R1 the 2 IP subnets ( and are present in EIGRP routing protocol.


On R2, there is already a basic redistribution between OSPF and EIGRP. Also notice that the IP subnet which connect R2 and R3 is routed in OSPF area 0 (zero)


As well on R3, the IP subnet between R2 and R3 is present in OSPF area 0 (zero) and the other 2 subnets ( /24 and /24) are in OSPF area 1 (one). Now, if you remember from my older post, if you have a Loopback interface with a IP address (doesn’t matter what netmask) this will be always advertised in OSPF as /32. I did a little trick to be sure that they are still advertised in OSPF as presented under Loopack 1 ( /24) and Loopback 2 ( /24). You want to know how, check this post.

Until now, we saw how the routers are configured. Let’s have a look how the routing table appear now on this routers.

Among other routes, please notice that we receive the 2 networks from R3 each of them with a /24 prefix. Keep this in mind as it’s important for later.


On R2 we have both the EIGRP and OSPF routes, each with a /24 prefix. In some minutes we will change this. Let’s check the last router.


Here we have the 2 prefixes from EIGRP domain, again with /24.

After reviewing all the necessary information let’s apply the configuration. I will start with Area range command. For example in our case we want to advertise only one subnet from Area 1 to Area 0 in OSPF. We will combine the 2 x /24 subnets in one /22.
Why /22 and not /23? Because we have /24 and /24 and will not be a valid prefix. Keep in mind that you have to stick to the subnetting rules. Indeed with this /22 we will “catch” also and in our range, but being in a test environment this is  not a problem for now. In the real world you have to take care about discontinuous networks and to apply summarization only when it’s possible.

On  our R3 router we should apply the following configuration:

configure terminal
router ospf 1
area 1 range

If we check now R2, we should see:

Only one /22 subnet. IP subnet summarization is successfully taking place.

What about Summary address command? Remember that we redistribute from EIGRP into OSPF, so if you have a look about, right now in the OSPF cloud we have 2 /24 subnets imported from EIGRP domain. We want to summarize this 2 addresses in a /22 (the same reason like explained above). For this we will use the Summary address command. In short explanation, this command is only used on ASBR routers, when you want to summarize IP subnets imported from a non-OSPF domain.
On R2, we a apply the following configuration:

configure terminal
router ospf 1

Let’s check what we receive on R1 and R3 after summarization:

We could see clear the advantage. Summary route is present on R3 and again /22 is on R1. This is the nice part and we achieved what we wanted.

Very important!
What about the blue line? That a big problem. Maybe your idea was that I did some mistake when using .1. and .2. in the third octet the subnet. It was more easier to use .0. and .1. and then I could summarize easy with /23. But I wanted to show you a hidden danger which lies beyond the summarization. Remember that I said I did just a basic redistribution  between EIGRP and OSPF? Forget that! In test environment is OK, but in real ones not. Always try to reduce the amount of redistributed subnets between 2 domains to only the necessary one using route-maps or route tagging. This should be mandatory, when redistributed summarized networks which are discontinuous.

Back to the blue line. We redistribute from EIGRP to OSPF, where we did the summarization. Due to the fact that we used and we had to summarize to /22, which include also the non-present subnets and 192.168.3/24. Then we redistribute from OSPF to EIGRP. See the loop? EIGRP-OSPF-EIGRP. Since EIGRP see there a /22 prefix which include the 2 additional /24 subnets and of which R1 has no clue about, the router install this route in it’s routing table, thinking that R2 is the gateway for the 2 prefixes above. It sound complicated but I tried to simplify the explanation as much as I could. If we were using route-maps or route tagging this situation could be avoided.

10 tips that I can tell you about Cisco CCIE exam preparation

10 tips Since now I’m closer than ever to my CCIE lab exam, I thought that it would be a good opportunity to share some of my preparation experience with you. Even if this post is focused of the CCIE preparation, I believe that you can take my advices into consideration even if you are preparing for some other exam.

As most of you, when I started the preparation I searched on the Internet about some advices like what should I read, how much time to dedicate to study and how to achieve the maxium results. This are only a few example, the entire list is much more longer and boring I believe.

I found a lot of tips, tricks, advices and many more. Some of them were really useful and helped me, but a lot (and I mean a lot) had no idea what they are talking about. I was dissapointed to find out that some advices there were just lines on a web page and the author had no idea what he or she was talking about, just taking the ideas from another blog (usually one which belong to a network engineer) and posting on his or her blog, webpage article.

So, why I’m writing this post. Well I hope that the way I see things and the advices that I give based on my personal experience can really help some of you.

Don’t trust everything and everybody – If somebody tell you that you should read at least xy number of books and take 1-2-3 bootcamps, stop one second and think. Compare your experience with the one of the person you are speaking to. Maybe you have 7-10 years of experience in related field and you already know most of the theoretic things. Don’t spend time reading just because somebody praise himself on mailing list or some forum with how many materials he read.

Don’t get discouraged – If you are reading on Internet (e.g. forum, mailing lists) ideas  that get you discouraged on and on…quit reading them; YOU are the only one aware of your knowledge and you have to trust yourself not opinions on the Internet. Just because somebody say that you will fail on your first attempt, does not mean that it will be like that.

Do not learn if you are tired – You will only get more tired and more frustrated. Do not compare the time you are learning with the one of other CCIE pretenders. You don’t have to learn 12 hours / day just because somebody does it. Maybe that persons has not other thing to do, no work, family, pets  or he / she are more slow learners than you. It’s not a competition who’s learning more in a day. If you are tired after a work day, just go to bed, sleep and then when you can you will learn. You’ll see that you can learn in 2 hours when you’re rest more that in 4 hours when your tired.

Don’t quit, just relax – If you encounter some problems during your lab preparation, and you just cannot see the solution, take a 30 minutes break, and maybe than you can see the task with “different eyes”. Sometime the solution is just in front of you, but you cannot see it right away.

Trust your knowledge – I’m not saying here to solve one lab or task during preparation and to praise yourself being so smart, but the right imagine about you can help. Remember that the more trustful you are in our skills, the more chances to obtain a positive results you have.

Don’t hesitate – When you are doing labs, task, questionnaires usually the first idea you have is the best one. Analyze the request correct but don’t start questioning yourself if are doing well, maybe you should apply other solution, or maybe…all of this will confuse you and give you headache. At least now you are in preparation and you can check if you are thinking in the right way. Also you can learn from your mistakes.

When you are not learning try not to think that you should do it – This is one of the most hard thing to achieve, and I have to admit that in most of the time I cannot follow this advice. But if you can, do it. As an example, you really want to see movie, you go to the cinema, but your mind is just making you think of networking, topologies, cisco and so on…At the end you will see that you did not enjoy the movie at all, didn’t relax and didn’t learn either. You gain nothing.

Do not neglect your closer ones during preparation – You will see that you have much more to earn if you give up 1-2 hours of learning in weekend and get out with your family, girlfriend, friends or pet. Try to understand that you have more to gain from their support sometimes than from those 2 extra hours of learning.

Prepare yourself – In a consistent and rigorous mode. After all any exam and especially Cisco CCIE exams are hard to pass. This kind of exams can really improve your professional and personal life.

Be kind and share knowledge to those who are in need – You never know when somebody will return the favor to you. I’m not saying here to let somebody (even more if that somebody is making money from your help) to take advantage of you, but helping make you feel better and prove to you that you didn’t learn for nothing.

Maybe you expected more technical advices and tips and you are a little bit disappointed, but I can assure you that the lines above can really help you in your preparation. Nothing is more important that to understand that CCIE exam is design to help you improve your life not destroy it. If you neglect everything in favor of preparation and then you have bad luck and do not pass it from the first attempt, you could become frustrated because you’ll realize that you spent a long time just learning and now you have nothing. This will stop you from going to the second or third attempt and you’ll end with lost time and without any degree.