Impact of auto-summarization on a discontiguous network

Unanswered Question

A network audit recently found 48 of 155 routers using eigrp configured with the default auto-summary. They are contained within a single eigrp AS and the network seems to work fine, most of the time. What's the potential impact of having 31% of our routers with auto-summarization...most, not all, are stubbed... on mixed frame-relay, metro E and fiber network. Is there a way to measure the impact before and after these are set to no auto-summary? Andy

I have this problem too.
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sundar.palaniappan Tue, 05/13/2008 - 15:38

It's hard to say precisely how much of an impact it could be causing in your network unless we know you complete topology and configuration. However, it's not advisable to leave auto-summary enabled despite the fact you aren't having any issues in your network. It's quite possible there may be sub-optimal routing in your network - traceroute between subnets should help identify this. Moreover, when subnet(s) go down some traffic might be getting to remote routers and could be getting dropped there and thus resulting in wasted use of bandwidth. If you can present more information on your setup it would help identify the issues more accurately rather than a generic comment.



ralphcarter Tue, 05/13/2008 - 18:27

Be careful when and if you plan on removing auto-summary.

adding or removing auto-summary will clear out your EIGRP Process and momentarily bring down your network.

Richard Burts Tue, 05/13/2008 - 18:33


In my early days of working with Cisco equipment I tended to believe that the default behavior of enabling auto-summary was a good thing and was the norm. (and in those days most of the networks that I dealt with were contiguous) As I gained more experience I began to see more and more networks that were not contiguous and for which the default of auto-summary poses a problem. Increasingly I regard no auto-summary as the default posture.

I agree with Sundar that if we knew more about your network and its topology and its addressing scheme then we could give you better advice. So tell us a bit more about your network environment and its addressing plan.

Sundar discusses the issue as the possibility of sub-optimal routing. I will go a step farther and say that if there are parts of your network that are discontiguous and that have auto-summary enabled, then you have parts of your network that are broken and that have lost connectivity to some other parts of your network.

The problem will be made worse if you have the other old default enabled of no ip classless. In some networks with discontiguous networks, configured with ip classless and a good default route they can overcome the problem of discontiguous networks.




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