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I am currently working on an assignment that asks me to either aggregate four given IP addresses or state why they cannot be aggregated. I did calculate an IP address from which I believe that it summarizes the given four addresses, however, I am sure there's a catch. They wouldn't ask so shady if there wasn't. How can I find out whether I have a set of addresses that cannot be summarized? Could you provide a concrete example?

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    For future reference, homework or other "assignment" questions are off topic. – John Jensen May 21 '15 at 23:15
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If the aggregate network is not contiguously used then aggregating the subnets would hijack and blackhole some traffic. For example:

10.1.0.0/24 - Site A
10.1.1.0/24 - Site A
10.1.2.0/24 - Site B
10.1.3.0/24 - Site A

Aggregating 10.1.0.0/22 to Site A would steal the 10.1.2.0/24 traffic from Site B and route it to Site A. That's not to say you can't aggregate the traffic, but doing so would be unwise.

Also a large disparity in the subnets would make them unfeasible to aggregate, 172.16.1.0/24 and 192.168.1.0/24 for instance.

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  • Now, you say it's "unwise" to aggregate these four addresses. However, when I use the common way to calculate an aggregation route, I get 10.1.0.0/22--which looks fine to me. How can I see that this is wrong? – user15544 May 21 '15 at 17:14
  • From a math perspective it is correct. From a real-world perspective you would need to verify that all the routes you want to aggregate at "Site A" actually originate from "Site A" by looking at the routing tables. – Robert May 21 '15 at 17:19
  • Actually, I don't want to aggregate to Site A. Let's say I have a Router 1 which knows all 4 destinations. Then I have another Router B, and I would tell it to route all traffic matching 10.1.0.0/22 to Router A instead of worrying about the 4 destinations. Would that work? – user15544 May 21 '15 at 17:23
  • If you're using a dynamic routing protocol you would create the Aggregate at Router 1, so that it is advertising a single aggregate prefix instead of 4 smaller networks. If you're using static routes then you would create the larger static route at Router B like you said. – Robert May 21 '15 at 17:29
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    As long as site B is still announced, it's more specific /24 would be higher priority than a /22. – Ricky Beam May 21 '15 at 20:40

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