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New Member

Automatic Address Summarization


I have a question.

1. Is it a good idea to disable auto summarization on routers. ?

Hope someone can advice. Thanks in Advance




Re: Automatic Address Summarization

It depends. If you have a lot of specific routes in your table, automatic summarization can help aleviate the bandwidth usage on your serial links for instance. In the case of distance-vector routing protocols ( which send updates on a regular basis ) this saves the quantity of routing updates traversing your link.

Summarizing networks hides more specific networks and could lead to unintentionally blackholing traffic. This is specifcally the case when you are dealing with discontiguous networks.



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Re: Automatic Address Summarization

Hi. it's depends on your network

let take one example

if you have network, , then you can summarization this two network into single network etc.

this will help to reduce routing entry in other routers

when you disable auto summarization ,all you network will prapogated induvidually.

i hope this will help you out to understand route summarization.


New Member

Re: Automatic Address Summarization

First - you cannot enable or disable auto-summarisation on routers.

You can configure Cisco routers as 'classless' or not - implying that they understand that prefixes are independent of any class-based structure (A,B,C).

Auto-summarisation is a function of a routing protocol. The simple answer to the question is NO it is NOT a good idea disable auto summarisation. (Oh, I can hear the techies jumping up and down at that).

Reason? with no other contributing factors except the question as to whether it's a good idea, the only possible response is No.

But the 'fully-qualified' response is this... if your network and network-population is addressed in a proper Classful manner where all networks are contiguous, then there is value in having routing protocols configured to perform auto-summarisation.

But there are most likely no networks that look like that. And as soon as you do break a natural network, you now have multiple natural summary's floating around your network pulling traffic in different paths to different destinations. Not a good situation.

It's a big question with a lot of the base elements of good IP address structure at its core. You should go read all about CIDR, and understand the differences between a prefix, a network and a subnet.


New Member

Re: Automatic Address Summarization

Thanks for all the advice and replies.

If I am not wrong, no auto-summary can disable auto summarization.

Correct me if I am wrong. :)


Alan Chua

New Member

Re: Automatic Address Summarization

You are correct.

And, it is not a global configuration command.

It is a routing protocol sub-command.