Forum Replies

  1. Rene,

    Thank you so much for your attention!!! Now I can understand better about this subject!

    and about:

    “The only thing to be aware of is that your summaries include networks that you “don’t have”.”

    Yes, There is situation where is not possible to have a summarization so specific like example above, where we had that summarize 5 networks and it was need to use a block size of 8.

    I’m really grateful/thankful with your explanations. Thanks, thanks and thanks!!!


  2. Hi James,

    Where did you find this example? It’s not on this page? :slight_smile:


  3. Rene,
    I got it off a ccna test page, but I believe I figured out the solution, what threw me off what the, it’s similar to how you solved the other two.


  4. Hi Lokesh,

    There are a couple of potential issues.

    1. You can blackhole traffic. For example let’s say you have these 4 networks behind a router: /24 /24 /24 /24

    If you would create a summary like /20 then basically you are advertising the - range to other routers. When your router receives a packet with destination or something it will drop it.

    1. Asymmetric routing, This one is harder to explain with text only. When you advertise a summary it’s possible that other routers will select a different (more specific) path as the next hop.

    2. Traffic engineering: If you use a routing protocol then you can play with the different metrics for each prefix. If you advertise a summary instead of all the different prefixes then you won’t be able to change metric (or BGP attributes) for a single prefix.


  5. andrew says:

    CIDR and Supernetting are the same thing. This is where you can aggregate networks together into larger networks beyond their natural network boundary.

    Although Supernetting/CIDR might be considered a type of summarization, “summarization” generally is considered to be constrained by natural (classful) network boundaries. The distinction is somewhat subtle. Suppose you have the following:

    These could be summarized as because /27 is smaller than the natural /24 boundary.

    Now, if you had this:

    The supernet (or CIDR) of these would be because /23 is greater than the natural /24 boundary.

25 more replies! Ask a question or join the discussion by visiting our Community Forum