 # Route Summarization

This topic is to discuss the following lesson:

Rene,

I have a doubt, all examples that you gave are “continuos” networks and an even number of networks.
And when we have networks like below? I just can solve them with binary method. Is there another form?

172.16.10.0/24
172.16.20.0/24
172.16.30.0/24
172.16.40.0/24
172.16.50.0/24

I choosed shortest and highest networks and convert them to binary, so the summary address will be
176.16.0.0/18 a block size 64 networks. I can’t solve it using CIDR notation and block size method.

Another example with an odd number of networks, I can solve it with binary method

192.168.0.0 / 24
192.168.1.0 / 24
192.168.2.0 / 24
192.168.3.0 / 24
192.168.4.0 / 24

The summary address will be 192.168.0.0/21, but when I was writing this example I saw that if I use block
size it’s able solve it too. It’s need always think in block size like “powers of 2”.

When you have a free time, please, detail below

8 + 8 + 6 = 24 bits

Hug and thanks for yours articles. It’s helping a lot 1 Like

Hi Gabriel,

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

172.16.10.0/24
172.16.20.0/24
172.16.30.0/24
172.16.40.0/24
172.16.50.0/24

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 172.16.0.0.

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

2,4,8,16,32,64,128.

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 255.255.192.0. 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:

172.16.0.0/24 = 1 network
172.16.0.0/23 = 2 networks
172.16.0.0/22 = 4 networks
172.16.0.0/21 = 8 networks
172.16.0.0/20 = 16 networks
172.16.0.0/19 = 32 networks
172.16.0.0/18 = 64 networks

Now you can see that 172.16.0.0/18 is the summary that will include all those networks that you want…this is the quickest method.

Let’s look at the second example:

192.168.0.0 / 24
192.168.1.0 / 24
192.168.2.0 / 24
192.168.3.0 / 24
192.168.4.0 / 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 255.255.248.0

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:

192.168.0.0/24 = 1 network
192.168.0.0/23 = 2 networks
192.168.0.0/22 = 4 networks
192.168.0.0/21 = 8 networks.

So we’ll go for 192.168.0.0/21 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”.

Rene

2 Likes

Rene,

“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!!!

Hug

Hey Rene,
I was following you example but got confused on these:
172.1.4.0/25 10101100.00000001.00000100.00000000
172.1.128.0/25 10101100.00000001.10000000.00000000
172.1.5.0/24 10101100.00000001.00000101.00000000
172.1.6.0/24 10101100.00000001.00000110.00000000
172.1.7.0/24 10101100.00000001.00000111.00000000

Particularly the 2nd one 172.1.128.0/25. 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.

thanks
James

Hi James,

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

Rene,
I got it off a ccna test page, but I believe I figured out the solution, what threw me off what the 172.1.128.0/25, it’s similar to how you solved the other two.

thanks
James

Ah I see…they probably included it to throw you off guard You can summarize 4,5,6 and 7 to 172.16.4.0 /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:

192.168.0.0 /24
192.168.1.0 /24
192.168.2.0 /24
192.168.3.0 /24

If you would create a summary like 192.168.0.0 /20 then basically you are advertising the 192.168.0.0 - 192.168.15.0 range to other routers. When your router receives a packet with destination 192.168.6.6 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.

Rene

For #1, why will router receive traffic for 192.168.6.6 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 192.168.0.0 to 192.168.15.255 fall under the 192.168.0.0 /20 summary so if you advertise that summary in a routing protocol, other routers will use it.

192.168.6.6 falls within that range so if another router doesn’t have a more specific route than the 192.168.0.0 /20 summary it will use it.

1 Like

hi

ip range 126.89.120.0/22

how many sunbets will that cover and what ranges

Hi Aaron,

126.89.120.0 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 126.89.120.0, 126.89.121.0, 126.89.122.0 and 126.89.123.0

Rene

so will it cover
126.89.120.0/27
126.89.120.64/27
126.89.120.96.27

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 126.89.120.0/22 to all eigrp neighnours to form relationships

and only advertise 126.89.120.0/22 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?