Complex Route Summarization
Thursday, 17 April 2008
I've already posted about route summarization but I found this interesting technique for calculating complex route summaries at http://www.sadikhov.com/forum from member MarkinManchester. Thanks Mark!
Sometimes you are asked to filter a complex set of routes using as few lines as possible. Here we will examine a way to calculate the subnet and wildcard for such a scenario. With some practice, you should be able to calculate any set in 5 minutes or less. It is also recommend to test your results in a lab setting, using loopbacks and distribute-lists.
For example, say we are asked to filter the following routes using a one-line filter:
102.17.63.0
126.22.61.0
111.22.57.0
125.33.101.0
First, make a table as shown:
Then, enter the decimal digits from the first octet in the left-hand column. Convert each number to binary. You can use Windows Calculator set to Scientific Mode to do the conversion if you so wish.
In the binary area, first check the octet then column by column, apply the following rules:
- If the entire octet is 0, the subnet is 0 for that octet, and the mask is 255 for that octet.
- Else If the column is all 0’s, the subnet is 0, and the mask is 0
- Else If the column is all 1’s, the subnet is 1, and the mask is 0
- Else the column is a mixture of 1’s and 0’s, the subnet is 0, and the mask is 1.
For the first octet the table should look like:
Convert the binary Subnet and Wildcard Mask to decimal to complete the table:
So far, our filter looks like: 100.xxx.xxx.xxx 27.xxx.xxx.xxx
For the second octet the completed table should look like:
Our summary address now looks like: 100.0.xxx.xxx 27.55.xxx.xxx
For the third octet the completed table should look like:
Our summary address now looks like: 100.0.33.xxx 27.55.94.xxx
Now the last octet for all addresses is zero so the subnet address must be 0 in the last octet and the wilcard mask must be 255 in the last octet.
Our completed summary address looks like: 100.0.33.0 27.55.94.255
Apply this technique as you wish, for example:
access-list 1 permit 100.0.33.0 27.55.94.255
This is a great technique for reducing something complex into easier computation so hats off to Mark again for turning me on to this.
Posted byChris Bloomfield at 19:47 5 comments
Labels: complex, Route Summarization, Summarisation, summary routes, Supernetting, wildcard
Route Summarization
Friday, 23 November 2007
Some people get really confused when it comes to route summarization, probably more so with the different names given to it:
Route Summarization
Supernetting
Aggregate Address
Summary Address
Actually, I get more confused typing out this thing as being English I want to replace the z in "summarization" with an s, but as it's Cisco I best keep the z in, lol. I also get this with "neighbor" versus "neighbour".
Anyway, I digress. The above terms all mean the same thing.
What is summarization?
The process of taking a range of IP addresses and advertising them in one address block. The most well-known summarization/supernet is the RFC1918 Class B Range. More commonly we know the range to be 172.16.0.0 - 172.31.255.255, however the supernet is 172.16.0.0/12. You see what has happened there? We've taken a range of addresses and squashed it into one advertisement.
What are the benefits of summarization?
Quite clearly, if we have just one address instead of lots of individual addresses then the routing table is going to be smaller. This in turn means that memory requirements are reduced.
The less obvious benefit is that summarization means you're tracking whether or not you're connected to some subnets of a summary, not the up/down state of every link. Thus when the link goes up or down, you don't have a flurry of traffic announcing the state change.*
How to summarize/supernet?
This is what you really want to know isn't it? Well it's dead simple.
The first method shows you the long way.
1. Starting from the left of the IP address, identify the first octet that has a change of address in it. For example, the Class B RFC1918 range, the IP address first changes in the second octet (i.e. 172.16.x.x - 172.31.x.x).
2. Write out the binary equivalent of the address up to and including the changing octet. So for example the range above is:
10101100.00010000 = 172.16
10101100.00010001 = 172.17
10101100.00010010 = 172.18
10101100.00010011 = 172.19
10101100.00010100 = 172.20
10101100.00010101 = 172.21
10101100.00010110 = 172.22
10101100.00010111 = 172.23
10101100.00011000 = 172.24
10101100.00011001 = 172.25
10101100.00011010 = 172.26
10101100.00011011 = 172.27
10101100.00011100 = 172.28
10101100.00011101 = 172.29
10101100.00011110 = 172.30
10101100.00011111 = 172.31
3. From this list, count from the left how many bits are the same in each address. If we look at it we see that the first 12 bits for each address is the same so that gives us our mask in slash notation. We therefore start at our first address 172.16.0.0 and append our mask so the summary address is 172.16.0.0/12.
Well that was quite simple. But can we be quicker? Yes we can is the good news, after all, you don't want to eat up time in the exam by writing out addresses in binary. So here goes:
1. How many subnets are in the range? The RFC1918 Class B range is 16 subnets.
2. What power of 2 equals our range? 16 subnets = 24 so the answer is four.
3. Subtract the figure from step 2 from the default mask of our address range. In this example our default mask is 16 so the mask after subtracting 4 is /12.
4. Add this mask to the first address in the range - 172.16.0.0/12 in this example
Quick, eh? This is how it works in my head, "mmmm, 16 addresses, 2 to the 4 is 16, mask is 16, minus 4 is 12, so summary address must be first address with /12 mask."
One last example:
Summarise the following:
192.168.0.0/24
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24
There are 4 subnets. 2 to the power of 2 gives us 4 so default mask of 24, minus 2, gives us /22. Address is therefore 192.168.0.0/22.
More difficult supernetting question
There are, however, a few pitfalls with supernetting/summarization. Sometimes you may not be able to get all of the addresses into a supernet without wasting addresses. Experience with supernetting questions will help you to identify this. An example below shows you how this could happen:
You need to summarize the following range:
10.16.31.0/24
10.16.32.0/24
10.16.33.0/24
10.16.34.0/24
10.16.35.0/24
10.16.36.0/24
10.16.37.0/24
10.16.38.0/24
10.16.39.0/24
10.16.40.0/24
What summary address should you use? Well there's 10 subnets. 2 to the power of 3 only gives us 8 subnets which is too small so we need to look at 2 to the power of 4 which is 16. The problem is that if we count in 16s our addresses straddle two subnets: 10.16.16.0 to 10.16.31.255 and 10.16.32.0 to 10.16.47.255. We have the same problem if we use the power of 5 giving us a range of 32 addresses (10.16.31.0 is in a different subnet than the other addresses in the range). We therefore have to go out to 2 to the power of 6 = 64 in order to get all of our addresses in the range (i.e. 10.16.0.0 to 10.16.63.255). What a waste of address space!!
The best answer is the following:
Break the space down into three ranges. We can summarize 10.16.32.0 to 10.16.39.255 with 3 bits (i.e. there are 8 subnets and 2 to the power of 3 is 8). We simply leave the other two address as they are. We have therefore avoided any wasted address space.
If you are unsure that you have the correct answers why not download a subnet calculator to double-check your answers? There is a great one by 3Com and can be downloaded from here.
I hope this has helped you with route summarization.
Chris
*http://www.netcraftsmen.net/welcher/papers/ospf2.htm
Posted byChris Bloomfield at 09:43 80 comments
Labels: Aggregate, Route Summarization, Summarisation, Supernetting, VLSM
