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Route Summarization

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Route Summarization


There are many millions of routes on the internet. If these routes all had to be stored individually the internet would have come to a stop many years ago.

Route summarization is also knows as supernetting and was proposed in RFC 1338 which you can read online by clicking on the RFC or if you have printed this document by visiting If you want to read a very comprehensive guide to route summarization then please grab a hold of Jeff Doyles excelling Cisco book Routing TCP/IP Volume 1 which is in its second edition now.

ZIP Codes

ZIP codes were used by the United States Postal Service to improve routing of letters to addresses within the USA. The first digit represents a group of US states and then the second and third digits represent a region inside that group. The idea is that letters and parcels can be quickly routed by machine or hand into the correct state and then forwarded to that state.

When it reaches the state it can be routed to the correct region. From there it can be routed to the correct city and so on until it is sorted into the correct mail bag for the local postman or lady.


The system was devised to make routing of mail more accurate and efficient. The sorting office in Atlanta doesn't need to know which street in San Francisco the packet is destined for. Having to store that information would make the sorting process unworkable.

Route Summarization Pre-requisites

In order to use route summarization on your network you need to use a classless protocol such as RIPv2, EIGRP or OSPF. You also need to design your network in a hierarchical order which will require careful planning and design. This means that you can't randomly assign networks to various routers or LANs within your network.

IP Addresses

See what I did there? I cleverly used the example of letters and will now apply it to IP routes. Well, in fact I think I have made my point so I won't labour the point. Let's move onto an example of a network and what the problem will look like on your network if you don't use route summarization.

Here is an example of how summarization would work with a range of IP addresses on a network.

The router has several networks attached. The first choice is to advertise all of these networks to the next hop router. The alternative is to summarize these eight networks down into one route and send that summary to the next hop router which will cut down on bandwidth, CPU and memory requirements.

You can only really work out a summary route by converting the IP address into binary (sorry). If you don't do this then you have no way of knowing if you are advertising the correct summary route which will lead to problems on your network.

Firstly, write out all of the network addresses in full and then the binary versions to the right of that. 10101100.00010000.00001000.00000000 10101100.00010000.00001001.00000000 10101100.00010000.00001010.00000000 10101100.00010000.00001011.00000000 10101100.00010000.00001100.00000000 10101100.00010000.00001101.00000000 10101100.00010000.00001110.00000000 10101100.00010000.00001111.00000000
Matching Bits 10101100.00010000.00001 = 21 bits

I have italicised the bits in each address which match. You can see that the first 21 bits match on every address so your summarized route can reflect these 21 bits:

One other significant advantage of using route summarization is that if a local network on your router goes down, the summary network still be advertised out. This means that the rest of the network will not need to update its routing tables or worse still, have to deal with a flapping route.

Exercise 1

Please write out the binary equivalents for the below addresses and then determine which bits match. I have written the first two octets for you to save time. 10101100.00010000. 10101100.00010000. 10101100.00010000. 10101100.00010000. 10101100.00010000. 10101100.00010000. 10101100.00010000. 10101100.00010000.

What summarized address would you advertise?

I make it or /17

Exercise 2

The below company has three routers connecting to their HQ router. They need to summarize the routes advertised from London 1, 2 and 3.

Let's start with London 1. 00001010.00000001.00000000.00000000 00001010.00000001.00000001.00000000 00001010.00000001.00000010.00000000 00001010.00000001.00000011.00000000 00001010.00000001.00000100.00000000 00001010.00000001.00000101.00000000 00001010.00000001.00000110.00000000 00001010.00000001.00000111.00000000

We have 21 common bits so London 1 can advertise /21 to HQ router.

And for London 2. 00001010.00000001.00001000.00000000 00001010.00000001.00001001.00000000 00001010.00000001.00001010.00000000 00001010.00000001.00001011.00000000 00001010.00000001.00001100.00000000 00001010.00000001.00001101.00000000 00001010.00000001.00001110.00000000 00001010.00000001.00001111.00000000

London 2 also has 21 common bits so can advertise /21 to the HQ router.

And onto London 3. 00001010.00000001.00010000.00000000 00001010.00000001.00010001.00000000 00001010.00000001.0001010.00000000 00001010.00000001.00010011.00000000 00001010.00000001.00010100.00000000 00001010.00000001.00010101.00000000 00001010.00000001.00010110.00000000 00001010.00000001.00010111.00000000

London 3 has 21 common bits also we can advertise upstream to HQ router.

In Conclusion

You will be expected to understand route summarization for the CCNA exam. If you can quickly work out the common bits then you should be able to answer the question quickly and accurately. If you want to read up to learn more about route summarization then please get yourself a copy of Jeff Doyles book Routing TCP/IP Vol 1 2nd Edition.

Here is the answer to Exercise 1:

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