Route Summarization

Hi Gabriel,

Good question, let’s look at these examples. First one:

Let’s do it in binary first (in case someone else reads this):

10 = 00001010
20 = 00010100
30 = 00011110
40 = 00101000
50 = 00110010

Only the first 2 bits are the same. Our CIDR notation would be 8 + 8 + 2 = 18 bits and we’ll use network address

This works but it’s slow…you can do it in decimal, just remember the block sizes:


Now you only have to pick a block size that fits all of the networks that you want. The only block size that fits your networks is 64 or 128. We’ll try to be as specific as possible so we’ll go for the 64.

Now you only have to figure out the subnet mask, just use this trick:

256 - block size = subnet mask.

So that’ll be 256 - 64 = 192. The subnet mask will be You can calculate the subnet mask back to the CIDR notation…

255 = 8
255 = 8
192 = 2

So that’s a /18.

So in short, just “pick” a block size that matches all your networks and then figure out the subnet mask and/or CIDR notation.

The other quick method to look at it is like this: = 1 network = 2 networks = 4 networks = 8 networks = 16 networks = 32 networks = 64 networks

Now you can see that is the summary that will include all those networks that you want…this is the quickest method.

Let’s look at the second example: / 24 / 24 / 24 / 24 / 24

Let’s do the “block size” method first. You can choose between 2,4,8,16,32,64 or 128. 2 and 4 are too small so we’ll go for 8.

256 - 8 = 248 so the subnetmask will be

255 = 8 bits
255 = 8 bits
248 = 5 bits

So the CIDR notation is /21.

The other quick method to look at this is like this: = 1 network = 2 networks = 4 networks = 8 networks.

So we’ll go for as our summary. This is the quickest method to find the summary address.

Hope this helps! The only thing to be aware of is that your summaries include networks that you “don’t have”.




Thank you so much for your attention!!! Now I can understand better about this subject!

and about:

“The only thing to be aware of is that your summaries include networks that you “don’t have”.”

Yes, There is situation where is not possible to have a summarization so specific like example above, where we had that summarize 5 networks and it was need to use a block size of 8.

I’m really grateful/thankful with your explanations. Thanks, thanks and thanks!!!


Hey Rene,
I was following you example but got confused on these: 10101100.00000001.00000100.00000000 10101100.00000001.10000000.00000000 10101100.00000001.00000101.00000000 10101100.00000001.00000110.00000000 10101100.00000001.00000111.00000000

Particularly the 2nd one Following your example using the 3rd octet has a 128 in it, so how would I factor this in as the other octets are using the first 5 bits.


Hi James,

Where did you find this example? It’s not on this page? :slight_smile:


I got it off a ccna test page, but I believe I figured out the solution, what threw me off what the, it’s similar to how you solved the other two.


Ah I see…they probably included it to throw you off guard :slight_smile:

You can summarize 4,5,6 and 7 to /22

Hi Rene

What are the disadvantages of using Route Summarization? Can you explain those with some examples pl.

Hi Lokesh,

There are a couple of potential issues.

  1. You can blackhole traffic. For example let’s say you have these 4 networks behind a router: /24 /24 /24 /24

If you would create a summary like /20 then basically you are advertising the - range to other routers. When your router receives a packet with destination or something it will drop it.

  1. Asymmetric routing, This one is harder to explain with text only. When you advertise a summary it’s possible that other routers will select a different (more specific) path as the next hop.

  2. Traffic engineering: If you use a routing protocol then you can play with the different metrics for each prefix. If you advertise a summary instead of all the different prefixes then you won’t be able to change metric (or BGP attributes) for a single prefix.


For #1, why will router receive traffic for subnet, can you give me a topology or some conditions under which router is likely to receive traffic for other subnets unless it is designed in-correctly.

All addresses from to fall under the /20 summary so if you advertise that summary in a routing protocol, other routers will use it. falls within that range so if another router doesn’t have a more specific route than the /20 summary it will use it.



ip range

how many sunbets will that cover and what ranges

Hi Aaron, looks like this in binary:

01111110 01011001 01111000 00000000

Let’s look at the third octet, since that’s where 22nd bit is:

120 = 01111110 01011001 01111000
121 = 01111110 01011001 01111001
122 = 01111110 01011001 01111010
123 = 01111110 01011001 01111011

So there it is, it matches,, and


so will it cover

and so on ?

Yes, we are working with the third octet here so anything in the fourth octet is included.

so will it be good to advertise the summary route to all eigrp neighnours to form relationships

or advertise seperate subnets
and only advertise between bgp neighbours ?

Hi Aaron,

It depends…advertising a summary doesn’t have anything to do with neighbor adjacencies. Let’s say we have two routers…R1 and R2. R1 has 4 interfaces with a network on each interface.

When we advertise 4 subnets, R2 will have 4 entries. When an interface on R1 goes down then this will affect the routing table of R2.

If we would use a summary route instead of those 4 networks then whenever an interface goes down on R1, nothing will change on R2. This makes our routing table more stable.

So that’s the advantage of summarization, the disadvantage is that it can also influence your routing decisions. If you learn a more specific entry from another router then it’s possible that R2 doesn’t use the summary route anymore for certain destinations.

1 Like

what if you have 10 networks to summarize? would that fall under the 16-block?

The answer to this depends on size of the networks you are wanting to summarize and whether or not they are contiguous. For the sake of simplicity, let’s say you want to summarize the following into the smallest possible summary address:

When I do subnetting, I use a mental shortcut instead of going all the way down to binary. Instead, I used exponents using the following formula

2^X >= Y

This reads as “two to the Xth power greater than or equal to Y” where Y is the number of networks (or hosts) you are trying to solve for. X is the power of two necessary so that 2 to the Xth power is greater than or equal to the number of networks in question.

In our example, we are trying to solve for 10 networks (0 - 9). What power of 2 is necessary such that two to that number is greater than or equal to 10? The answer is 4, since 2^4 = 16. This gives us a value of X=4, but what exactly is X? X represents the number of bits that must be used, in this case, to represent the 10 networks for which we are summarizing.

This means that /20 mask ( is the smallest possible mask that could cover - So the summary address for 10 networks would be:

Note that it was NOT necessary to go all the way to a /16 summary in order to create the summary.


omg!!! rene you literally made me to understand summarization, just by looking at your example. i am so happy i understand it now, because i usually run away from summarization but your explanation nailed it for me. thank you so much. the fact you break things down to the simplest form, is priceless.

Thanks rene