How To: Router Summarization

Router summarization

Ripv1 and IGRP
those protocols do auto summary to the classfull network which means
if you try to add a network of 10.1.1.1 it will auto summary to 10.0.0.0

You cant turn off this auto summary in Ripv1 and IGRP

you can turn it off on Ripv2 and EIGRP summarizing

OSPF and IS IS does not auto summery

Note:
Classful routing protocols (RIPv1 and IGRP) automatically summarize routes on the classful network boundary and do not support summarization on any other bit boundaries. Classless routing protocols support summarization on any bit boundary.


Route Summarization Calculation example

suppose you got this networks attached to your router like different LAns

172.16.12.0/24

172.16.13.0/24

172.16.14.0/24

172.16.15.0/24

1) first step:
You will convert everything to binary

10101100.00010000.00001100.00000000 -->172.16.12.0
10101100.00010000.00001101.00000000 --->172.16.13.0
10101100.00010000.00001110.00000000 -->172.16.14.0
10101100.00010000.00001111.00000000 -->172.16.15.0


2) Second Step

You need to locate the bits till where the bits are equals
What i mean with this ?

10101100.00010000.00001100.00000000 -->172.16.12.0
10101100.00010000.00001101.00000000 --->172.16.13.0
10101100.00010000.00001110.00000000 -->172.16.14.0
10101100.00010000.00001111.00000000 -->172.16.15.0

As you can see all the bits are equal till 6th bit of the 3rd octet(can you can see it above i BOLDED all the equals bits)

Now you find this we will go to step 3

3)Third step

You need to count those bits, the summary route number will be the first IP address in the block, and the subnet mask will be the number of common bits.

so the answer is
172.16.12.0/22


Well i think this is all you need to know about summarizing