Route Summarization

Hi BAZ,
Yyup, bro. you can use route summarization or route filtering to reduce the number of routes in the routing table to reduce the CPU load.

You may also interested in this calculator @BAZ :)

https://asecuritysite.com/ip/routesum

Good answer

Hello there,

Route summarization can be done just by seeing how many bits are exactly the same as the prefixes that you are interested in summarize. Where the bits are the same, (always observing from left to right) it's revealed how long your mask needs to be in order to represent all prefixes you want. 

Example

10.10.16.0/24

10.10.17.0/24

10.10.18.0/24

10.10.19.0.24

In binary you have these addresses:

00001010.00001010.00010000.00000000

00001010.00001010.00010001.00000000

00001010.00001010.00010010.00000000

00001010.00001010.00010011.00000000

All digits in bold are exactly the same (22 digits), so now I know that I have to use a /22 mask. The address that I will use will be the first in the sequence, so it will be 10.10.16.0/22 in order to summarize those 4 networks. Note that the networks are in sequence (contiguous) and I represent exactly 4 networks using the /22. Let's assume that I had the 10.10.22.0/24 also and I wish to summarize them along with the others... If I reduce the mask to /21, I will also represent the addresses 10.10.20.0, 10.10.21.0, 10.10.23.0 alongside the 10.10.22.0/24. So if packets destined to these addresses were sent, they would match this 10.10.16.0/21 route. Therefore, it is not possible to summarize it in this manner. It's better to summarize the contiguous network as a /22 and keep announcing this specific network (10.10.22.0/24 in the example). 

You just got to be very conscious that if all prefixes are not contiguous, you will represent more networks than you actually have in your IP routing table, making it possible that traffic can be forwarded to the router where the network does not exist (creating a kind of blackhole).

In your example is not possible to see if these networks are contiguous.

If you are using BGP, just remember to enable the use of the as-set in order to keep an unordered list of the AS numbers of the networks that compose the summarized route (this will avoid a loop).  

In the BGP view, uses the following command:

aggregate ipv4-address { mask | mask-length } as-set

Thanks @bruno.guedes for sharing this.
Let me refine my issue " Please suggest some quick and accurate methods to do Summarization on *received Routes* (as we are receiving 20K+ routes "

Hello Baz,

I have never seen such a thing. Who is sending should advertise summarizing routes because once received, the only thing that you can do is a filter to discard routes that you don't want in your BGP routing table. You don't summarize received routes. I've never heard about it.
If you have to redistribute these routes to an IGP (like OSPF), then you could summarize in order to avoid your IGP having so many routes, but you will have these +20K routes in the BGP routing table anyway.