Route summarization

PlazmaPlazma Member Posts: 503
Ok so I have finally started my CCNP journey with the BSCI (woo?)

I started going over some things i struggled with back in the ccna (but conquered them) to refresh my memory on some things.. and one was route summarization.

I understand how to summarize many routes for the most part, there is only 2 things i get confused at

1. Over-summarization - is there a # value or "rule" that cisco uses to determine what they consider over-summarized? I mean i understand that if it includes a huge amount of extra addresses i tis over-summarized.. but i know how cisco asks questions and want to be 100% clear on it

2. For some reason, i cant wrap my head around summarizing routes with different subnet masks.

- example:

In summarization the subnet mask of the individual routes shouldn't make much difference should they? I think i am going in circles and constantly confusing myself.


  • mikearamamikearama Member Posts: 749
    Route summarization is an easy thing to over-complicate.

    In your example... in any example... look for what all the subnets in question have in common.

    You gave:

    and suggested

    It's the right summary address. Why? First, look at the mask (/21) in binary. /21 falls on the 5th bit (from the left) in the third octet... the "8". So this summary address packs 8 subnets into each summary address. Therefore: includes all the subnets from to That encompasses all of the subnets you listed.

    What if we went with or

    /20 ends on the 4th bit, which is the "16" in binary. So a /20 envelopes 16 subnets at a time ( to far more than you need.

    And the /22? It falls on the 6th bit, which is "4" in binary. So it would only encompass 4 subnets at a time. You would need to summary addresses then, since this one won't include

    Edit: I missed the part of your question about the masks of the original subnets. You're right, providing the other portions of the supernetted subnets are placed contiguously. It would suck if another piece of a /25 is on the other side of your LAN, and you just summarized it.

    So, the /21 is the right answer. Hope that helped,
    There are only 10 kinds of people... those who understand binary, and those that don't.

    CCIE Studies: Written passed: Jan 21/12 Lab Prep: Hours reading: 385. Hours labbing: 110

    Taking a time-out to add the CCVP. Capitalizing on a current IPT pilot project.
  • nice343nice343 Member Posts: 391
    Route Summarization Calculation example 
    suppose you got this networks attached to your router like different LAns     
    1) first step: 
    You will convert everything to binary 
    10101100.00010000.00001100.00000000 --> 
    10101100.00010000.00001101.00000000 ---> 
    10101100.00010000.00001110.00000000 --> 
    10101100.00010000.00001111.00000000 --> 
    2) Second Step 
    You need to locate the bits till where the bits are equals 
    What i mean with this ? 
    10101100.00010000.00001100.00000000 --> 
    10101100.00010000.00001101.00000000 ---> 
    10101100.00010000.00001110.00000000 --> 
    10101100.00010000.00001111.00000000 --> 
    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 
    Well i think this is all you need to know about summarizing If anyone find any mistake on my explanation they are welcome but im pretty sure its like this.
    My daily blog about IT and tech stuff
  • Paul BozPaul Boz Member Posts: 2,620 ■■■■■■■■□□
    I got a little carried away graphing this out but oh well.    /24 1000000.10101000.00000100.00000000    /24 1000000.10101000.00000101.00000000    /24 1000000.10101000.00000110.00000000  /26 1000000.10101000.00000111.00000000  /26 1000000.10101000.00000111.01000000  /26 1000000.10101000.00000111.10000000  /27 1000000.10101000.00000111.10100000  /27 1000000.10101000.00000111.11000000  /29 1000000.10101000.00000111.11001000  /29 1000000.10101000.00000111.11010000  /29 1000000.10101000.00000111.11011000  /30 1000000.10101000.00000111.11011100  /30 1000000.10101000.00000111.11100000  /30 1000000.10101000.00000111.11100100  /31 1000000.10101000.00000111.11100110  /31 1000000.10101000.00000111.11101000  /31 1000000.10101000.00000111.11101010

    In the above example, you have 17 individual routes. Rather than advertising 17 individual routes you can supernet them into one route. You do this by first converting the IP addresses to binary. You can see the binary representation to the right of the mask. If you read the binaries from left to right you'll notice that the first 22 bits are the same ( 1000000.10101000.000001xx). converted back to decimal this is You now have your supernet address. You need a mask to go with it however. To get this just put a 1 for every similar bit and a 0 when the columns stop matching. You wind up with 1111111.11111111.11111100.00000000, which when converted back to decimal, gives you a mask of, or /22.

    To recap,

    1.) Convert the addresses to be supernetted to binary.
    2.) Compare the bits from left to right and draw a line between the last bit that's similar for all addresses and the first bit that's different.
    3.) convert the similar bits to get your first address
    4.) for each similar bit put a one. for each dissimilar bit put a zero. Convert this to decimal for your mask.

    Now, to address your question regarding over-using supernets and being wasteful. I wouldn't so much look for a "cisco" answer, just use common sense. If you have a group of four /24 networks you'll ideally use a /22 mask because anything more would be grossly wasteful. You wouldn't use a /16 mask for a DHCP pool servicing 15 PC's would you?

    The main problem with supernetting is causing routing problems for others, IE claiming routes that do not belong to you because of poor supernetting. For this reason, supernetting on the internet should never include address space you don't own.

    Also keep in mind that when you implement supernets you're willingly taking out a certain level of granularity from your network. Supernets can hide downed or flapping routes, can provide for sub-optimal routing conditions, and in some cases even cause black holes and loops. You have to be careful how you use supernets.

    Also keep in mind that unless you initially design a network with a good addressing scheme you probably won't be able to efficiently use supernetting. Remember, it always starts with a good plan.
    CCNA Security | GSEC |GCFW | GCIH | GCIA
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  • PlazmaPlazma Member Posts: 503
    Thanks, i get it now!
  • Paul BozPaul Boz Member Posts: 2,620 ■■■■■■■■□□
    Hey man, I replied to your email. Sorry, I didn't see your PM here :)
    CCNA Security | GSEC |GCFW | GCIH | GCIA
    [email protected]
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