Route Summarization

Route Summarization: Route summarization is all about making the routing table smaller. The larger the routing table the inefficient the router becomes i.e when the packet
comes/goes out of the router the router has to check against all the routing table entries and find the best routes. route summarization shrinks the routing table. I want to illustrate route summarization
from the below figures

In the figure above router R1 sends all the 16 routes to router R2 and R2 sends all these routes to other corporate network if connected. Now R2 has
1> R2 has routes of all the destination that R2 has to anyway go to R1 to reach
2>If one of the network say 192.168.3.0/24 goes down then it needs to update and replicate to R2 and R2 has to replicate it to others.

Now, the question is Does R2 really need to know that 192.168.3.0/24 network is down. Well I dont think so, becoz when R2 sends packets to 192.168.3.0/24 networks it will sends these packets to
R1. Then R1 will reply with ICMP unreachable. What I mean to say is router R1 will take care of any packet going to that network.

Lets imagine if we can shrink the 16 routes of R1 somehow. This is done by route summarization. Route summarization summarizes all the routes of R1 to fewer advertisements
What we can summarize the route in R1 as 192.168.0.0/16( as now this contains all the 16 routes) network and now R1 advertises this route to R2. Now R1 supresses all the individual route.
But the problem here is If we summarize R1 route as 192.168.0.0/16 then we are wasting all the network beyond 192.168.15.0/24 and no other router can use other network in 192.168.0.0/16.
This is what I call inefficient summarization.

Efficient Summarization
Lets summarize this 192.168.0.0 ---- 192.168.0.15/24 network.

I have broken down all our networks in bits so that it will become easier to visualize the summarization process. The idea of route summarization is to take the bits that are similar/same between all of the routes and
grouped them together. Now if we see our routes
1st 192 octet is same ie 1st 8bits are same
2nd 168 octet is same ie 2nd 8 bits are same
and then 4 bits of third octet are same and from then it starts to differ. I have shown this by drawing a line
So We now have 20 bits same so the perfect route advertisement from R1 to R2 would be 192.168.0./20 is 255.255.240.0 S.M
192.168.0.0 255.255.240.0
11111111.11111111.11110000.00000000
 last increment bit = 16
 so our network will be
        192.168.0.0   ----------> 192.168.15.0
         192.168.16.0  ---------->192.168.31


Now if we reverse engineer the /20 subnet mask as shown above then we will find that /20 is fit for out 15 network route

what If one more network is added lets say one more network 192.168.16.0 is added. Now if we see just like from above the similar bits are 19 bits. So according to this we have to advertise 192.168.0.0/19 which will
give us 32 networks that means the router can use 192.168.0.0 --- > 192.168.31.0 networks. but we dont need networks above 192.168.19.0. ie networks beyond 192.168.19.0 is wasted. So the best way to deal with
this growth scenario would be to keep on advertising the 192.168.0.0/20 network and advertise 192.168.16.0/24 network as a seperate route until there is enoubh addition of networks to change ti to 192.168.0.0/19 network.


This is all about route summarization Hope you have understand this. Route summarization will be very helpful when we are doing link state routing protocols like ospf and eigrp.
Thanks and I'll hope to see you soon