Router Summarization Manually!

Summarization of Routes

Can be called 3 different things:

1. Route Aggregation
2. Route Summarization
3. Supernetting

How to manually calculated a summarized route:        /25
172.1..4.128   /25        /25          /24        /24

Binary Chart:

1 6 3 1 8 4 2 1  
2 4 2 6
0 0 0 0 0 1 0 0

172.1.(last in common octets)

                        22 23 24 placeholders

4.0          00000  1   0    0.00000000
4.128       00000  1   0    0.10000000
5.0          00000  1   0    1.00000000
6.0          00000  1   1    0.00000000
7.0          00000  1   1    1.00000000

*Notice the 22nd place holder is the last  in common. Choose this place.

To find the IP subnet and subnet mask to summarize this route, you use the 22nd placeholder for the subnet mask (/22). And you keep the number 4 value in the 3rd octet because it is the value for the 22nd placeholder.

Answer: /22

It is that simple. Do not try to make it harder than this…

Example from **** Great site!

Route Summarization Strategies

The following example illustrates a method of summarize a group of subnets.

1. Convert the addresses to binary
2. Find the all the common bits from left to right
3. Convert all the bits that are in common back to decimal, and leave all the rest of the bits to zero.
4. The number of bits that are in common will be their subnet mask 00001010 00000011 00000100 00000000 00001010 00000011 00000101 00000000 00001010 00000011 00000110 00000000 00001010 00000011 00000111 00000000

The bits in red are the common bits. When the bits in red are converted back to decimal, the end

result is This is the summary subnet.

To calculate the mask, count all the bits in red. There are 22 bits. Therefore the summary route is or subnet mask

Manual Route Summarization
Reduces the size of the routing table by aggregating a group of routes into a single route that is advertised to the other routers. As a result, there is speedier process in routing packets as well as less amount of processing power is consumed. Needs to be configured in order for it to occur to manually configure summary route with EIGRP (or RIP v2), use the interface subcommand ip

R1(config-if)#ip summary-address eigrp 1

R2(config-if)#ip summary-address eigrp 1




7 Responses to “Router Summarization Manually!”

  1. keep it up bells! yeah make it simple; i have to do it for myself 😉 it worked for me 😉 hammer it!

    Now this is what am talking about!!!! Thanks daddy!! As easy as 1 2 3 🙂 you are a star!!

  2. thanks Sir…hav been scratchin head for the concept..neva knew it was cake walk..made life lot easier..keep it up! thanks again!

  3. OMG! I have been struggling with every chapter so far in the CCNA book. However I finally made it to chapter 5; VLSM & Summarization. For months I have been so frustrated that I started to think I would never understand how to do any of this.

    I have spent at least the last 8 hours trying to understand route summarization when finally I found your page and after reading it the concept became clear to me.

    Thank you so much for explaining how to do Summarization in abc steps.

    Thank You,


    • you are welcome so many things are explained by people who make things difficult.
      it is not the fault of us stupid students (smile face) it is the job of the teacher
      to relay topics so we get it. you can do it Joanna!

  4. This is perfect example. I love this site to read rest of all topics as explaining fantastic.

  5. jay shah Says:

    thanks it daddy, now summarization becomes so easy for me
    Thanks a lot

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