JNCIE Tips from the Field :: Summarization Made Easy

Today we’ll start with a series of articles covering tips and techniques that might be utilized by JNCIE candidates, whether pursuing the JNCIE-SP, JNCIE-ENT, or even the JNCIE-SEC.  The tips and techniques I will be covering might prove to be useful during a lab attempt but could also be used in real-world scenarios to save time and minimize configuration burden in addition to eliminating mistakes that might otherwise be made.  I want everyone to understand that what I am about to write is simply a technique.  I am not divulging any materials or topics which are covered under NDA.

NOTE: For full disclosure, I must reveal that I am an employee of Juniper Networks in their Education Services department.  As such, I take the responsibility of protecting the content and integrity of the exam as well as the certification credentials very seriously.  I would never reveal anything which would allow a candidate to have in-depth knowledge of any specific topics or questions that may appear on the exam.  Not only that, I worked REALLY, REALLY hard to achieve my JNCIE certifications, and I believe everyone else should too! It’s certainly more rewarding that way too don’t you think?!

So without further delay, let’s take a look at today’s technique.

It is well known that sumarization is a key aspect of any type of practical exam involving routing of some sort.  Those who have ever taken a CCIE Routing & Switching or CCIE Service Provider exam can attest, summarization is one thing every expert level candidate needs to master.  It is no different with Juniper.  In fact, Juniper’s certification web page clearly lists as one of the JNCIE-ENT exam objectives the requrement to “Filter/summarize specific routes”.

What I will show you next is a technique which I find quite handy when attempting to determine the best summary for a given route, and you can do so without having to resort to pen and paper and figuring it out the old fashioned way, i.e. looking at prefixes in binary. This technique, rather, allows you to use the power of Junos to your advantage to perform these tasks.  What I will reveal will also show you a fundamental difference between IOS and Junos and highlights why I believe Junos to be a more flexible, powerful, and superior network operating system.  You simply can’t do what I am about to do on a Cisco platform running IOS.

So let’s start off by looking at a diagram.  Let’s say we have a network that has several OSPF areas, and we must summarize some information for each respective area towards the backbone without encompassing routing information that might exist outside of that area.


Here we can see we have a backbone area, consisting of two routers, P1 and P2.  P1 is acting as an ABR for Area 1 and is connected to both R1 and R2. P2 is acting as an ABR for Area 2 and is connected to R3.  As you can see from the diagram, I have configured more than a single set of IP addresses on many of the physical interfaces as well as the loopbacks.  This way I can represent many more networks and therefore create multiple Network LSAs for purposes of summarization.

So let’s assume that we need to create the largest aggregate possible for a particular area and advertise only that aggregate towards the core without encompassing any routes which might be outside the area from which the summary describes.  Now normally, one would take a look at the diagram, get out a pen and paper, and start a lengthy exercise of supernetting based on binary addresses.  This can take several minutes or more and is valuable time that could certainly be used on wide variety of other more important tasks like setting up MPLS LSPs or troubleshooting that Layer 2 VPN connectivity.  So let’s take a look at a simple trick that actually takes advantage of Junos to determine what the summary should be.

What we are going to do is take advantage of a feaure inside Junos which automatically shows us a range of prefixes which match a given CIDR block.  The Junos operating system has built-in route matching functionality which allows us to specify a given CIDR block and returns all routes with a mask length equal to or greater than that which is specified.  So by applying this principle, what we want to do is look at the diagram for a particular area, choose the lowest IP within that area as our base, and then apply a subnet mask to it which attempts to encompass that route as well as others.

For example, looking at this diagram, we see that the lowest IP address being used in Area 1 is the address assigned to R1’s loopback.  So let’s start by using this as our base for our summary, and then simply apply a subnet mask to it which we think might encompass additional routes:

Note: We can do this on any router within Area 1 since the Link-State Database is the same on all devices, but I prefer to perform the work on the ABR since this is where I will be performing the aggregation.  Also, the ABR may have other local and/or direct routes (or perhaps routes from other protocol sources) so we want to see things from the perspective of the ABR.

What we see here is that we have just now determined the summary route which in fact encompasses all the loopback addresses on both R1 as well as R2, but we need to keep going because this doesn’t incorporate the Gigabit Ethernet links between all the devices:

Not quite. Let’s keep trying:

Nope, still not there yet. Let’s try again:

Ok, this looks more like it.  Here we can see we have all the Gigabit Ethernet links connecting all devices, as well as the loopback addresses.  This might be a suitable summary.  Let’s keep going to see what happens:

Clearly from this command, we can see we have now gone beyond what might be considered a suitable summary because we are now encompassing routes that exist within the backbone Area 0.  So it should be clear from this simple set of commands that the would be a suitable address to use for our summary.

We could easily apply a similar example to Area 2 to quickly determine what the best summary would be.  We see from looking at the diagram the lowest IP within Area 2 is the loopback address applied to R3.  When we use that as our base and go through the steps above, we can find our summary:

And there you have it! As you can see it’s really quite simple and if you haven’t stumbled upon this already you may be saying to yourself, “Why didn’t I think of that before?”.  I hear from many candidates that they spend considerable time the old fashioned way to determine summaries and I always ask myself why.  As you can see, there is an easier way!

Clearly the benefit to using a technique such as the above is to easily find the routes that best summarize a bunch of more specific routes.  The utility of such an approach, while very useful during a practical exam, might be considerably lessened in the real-world where it is likely that hierarchy has already been built into the network and you have network diagrams at your disposal.  On the other hand, there may be situations where you inherit a network that was developed with hierarchy in mind, however summarization was never employed, or it was employed improperly.  In such cases, the above technique can be a real time saver, allowing you to spend less time doing binary math and more time doing the fun stuff – like troubleshooting why that MPLS LSP isn’t getting established!

Stay tuned for additional articles covering time saving tips and techniques which can be used during your next lab attempt!  Good luck, and may the force be with you!

13 Replies to “JNCIE Tips from the Field :: Summarization Made Easy”

  1. It is a really nice presentation!!
    But, there are some typo errors, am i right?

    “Area 1 is the address”, should be
    “Area 2 is the loopback”, should be

  2. SBS, thanks for the comments, yes I definitely see the typo – thank you very much for spotting this for me! I will go ahead and fix it right now. I should use you as a tech editor when I plan on writing my book 🙂

    Thanks everyone else for your kind and gracious comments.

  3. Hi! Regarding your comment on IOS: I’ve never touched JunOS but I don’t see how this differs from the “show ip route x.x.x.x x.x.x.x longer-prefix” command in IOS. Am I missing something here?

  4. Markku, you are not missing anything at all! This is the part where I take my foot out of the mouth and try to save face because yes, I was not aware of this particular command! This just goes to show that “supposed” experts can be wrong from time to time 🙂 Thanks for sharing this little tidbit and appreciate the insight!

    While I still feel that Juniper is the superior network operating system, this clearly illustrates that similar behavior exists in IOS.

    Thanks Markku for your insight and also for visiting my blog!

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