Route Summarization

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oute Summarization Friday, 23 November 2007 Some people get really confused when it comes to route summarization, probably m ore 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 r eplace the z in "summarization" with an s, but as it's Cisco I best keep the z i n, 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 addres s 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 address es 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 ever y link. Thus when the link goes up or down, you don't have a flurry of traffic a nnouncing 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 changi ng octet. So for example the range above is: 10101100.00010000 10101100.00010001 10101100.00010010 10101100.00010011 10101100.00010100 10101100.00010101 10101100.00010110 10101100.00010111 10101100.00011000 10101100.00011001 10101100.00011010 10101100.00011011 = = = = = = = = = = = = 172.16 172.17 172.18 172.19 172.20 172.21 172.22 172.23 172.24 172.25 172.26 172.27

10101100.00011100 10101100.00011101 10101100.00011110 10101100.00011111

= = = =

172.28 172.29 172.30 172.31

3. From this list, count from the left how many bits are the same in each addres s. If we look at it we see that the first 12 bits for each address is the same s o that gives us our mask in slash notation. We therefore start at our first addr ess 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. I n 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 examp le Quick, eh? This is how it works in my head, "mmmm, 16 addresses, 2 to the 4 is 1 6, 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 yo u may not be able to get all of the addresses into a supernet without wasting ad dresses. 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 powe r of 4 which is 16. The problem is that if we count in 16s our addresses straddl e two subnets: 10.16.16.0 to 10.16.31.255 and 10.16.32.0 to 10.16.47.255. We hav e the same problem if we use the power of 5 giving us a range of 32 addresses (1 0.16.31.0 is in a different subnet than the other addresses in the range). We th erefore have to go out to 2 to the power of 6 = 64 in order to get all of our ad dresses in the range (i.e. 10.16.0.0 to 10.16.63.255). What a waste of address s pace!! 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 simp ly 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 ca lculator to double-check your answers? There is a great one by 3Com and can be d ownloaded from here.

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