Route Summarization Shortcuts?

[Mike_B]

Hi Guys,

I'm just curious: What is the most efficient way to perform route summarization? Any tricks or shortcuts?
The method explained in CBT Nuggets is to:

1) Line up the similar network ids.
2) Find the first octet that is different (left to right).
3) Write out that octet's values in binary for the smallest and largest values.
4) Count how many bits are unchanged. Add that value to the number of other unchanged bits in the left octets.
5) Use that as your subnet mask (use necessary discretion to determine if the subnet includes too many unused networks)

So for example, lets say I wanted to create a summary route for the following networks:

192.168.1.0 /24
192.168.2.0 /24
192.168.3.0 /24
192.168.4.0 /24
192.168.5.0 /24
192.168.6.0 /24
192.168.7.0 /24
192.168.8.0 /24
192.168.9.0 /24
192.168.10.0 /24
192.168.11.0 /24
192.168.12.0 /24
192.168.13.0 /24
192.168.14.0 /24
192.168.15.0 /24

Step 2: I notice that the 3rd octet changes.

Step 3: I write out the octet values for the smallest and largest values.
192.168.00000000.0 /24
192.168.00001111.0 /24

Step 4: Count how many bits remain unchanged.
8 in the first octet
8 in the second octet
4 in the third octet
8 + 8 + 4 = 20

Step 5: Route summary is 192.168.0.0 /20

This works. But the problem is, it's kinda slow. Is there a faster way to do this?


Thanks,
M

[Scott Morris]

Memorize the bit boundaries? Although realistically, the more you work with binary the "slow and painful" way, the more it will simply become part of your thinking and you could do it off the top of your head!


Scott

[Mike_B]

Hi Scott,

Thanks for the prompt feedback. Can you expound a little on that? What do you mean memorize the bit boundaries?


-M

[Scott Morris]

Like 4 bits would be every 16'th number. 0, 16, 32, 48, etc. 5 bits would be every 32nd number. 0, 32, 64, 96, etc.

[ciscoskeemz]

Mike,

I learned the same way (by watching CBT Nuggets)....it does seem slow at first, but you'll definitely get faster at it...This is still my preferred method

[danieldsm]

Many thanks Mike.B for your helpful summarisation of your method for creating route summarisation shortcuts.
This really helped me understand and provided me with an easier method than the one I'd been trying to use before.


Cheers,
Daniel