EIGRP manual summarization

Unanswered Question
Feb 25th, 2009

If you have an existing connection that is manually summarising a network block as subnet, is it possible to use one of the subnets within that block for another remote site even though it is being advertised in the summary route or would it be easier to not summarise and increase the size if the routing table?

I have this problem too.
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simontibbitts Wed, 02/25/2009 - 12:33

Hello Keith.

Its not a good design to have a discontiguous network. Accurate summarization is the key to a good network.

However it is still possible to advertise another network if you have to. If for example you have a network elsewhere in the network, if you advertise it out so all other routers see it then you won't have a problem as the first routing decision is to choose the most specific route. a /24 address is more specific than a /18.


Giuseppe Larosa Wed, 02/25/2009 - 12:34

Hello Keith,

removing the summary route is a cleaner configuration and avoids to have traffic sent to the summary route when then new remote site is isolated.

To be noted that you can configure a new summary route more specific like where the new routes are not included.

Hope to help


Richard Burts Thu, 02/26/2009 - 20:04


We could give better answers if we had more details about your environment and what you plan to do.

But lacking that detail I would comment that my colleagues in their answers have not taken recognition that in routing decisions the longest prefix is always chosen.

So if I understand your question correctly you are planning to summarize a group of subnets. But there is a subnet that is part of that block and is not reached out the interface where the summary will be configured. In general this will work ok.

Assuming that the routing table of the routers in the network contain both the summary route to the block of addresses and contain the route to the more specific subnet then everything should work ok.

I would agree that it is not optimum design. But when the question is will this work or not, then the answer is that it will work.



keithannette Tue, 03/03/2009 - 08:29

Hi Rick,

Sorry for the late reply.

I already have 2 sites connected in a cloud. Manual summarisation is being used on the main interface to advertise the /18 routes. Unfortunately, I have to use the subnet inside this block for another site which will be connected to the 2 sites. I was wondering if I didnt use summarisation at all and had a larger routing table - would this be better than have a subnet summarised in more than one location. Which would you think would give a better design under the circumstances ?



Jon Marshall Tue, 03/03/2009 - 08:33


Personally i would keep the summarised routes and advertise out your more specific one as well rather than remove all summarisation.

There are some specific advantages to summarisation, one of the key ones being that a route change in one location does not necessarily have to be propogated throughout the network with the obvious savings in bandwidth, CPU and memory usage on your routers.


naveen_b81 Thu, 02/26/2009 - 21:39

You can certainly use the subnet within in antoher lcoation as long as it is not present in anywhere else. As the routing table lookup is always based on longest prefix match and hence the subnet route will take preference over the summarized route.

Mohamed Sobair Tue, 03/03/2009 - 08:44


This is neither optimul routing design, nor recommended setup.

However, its possible, the impact would be the longest prefix will always be preferred over the summarized route. Another impact would be if this Network (most specific route) always reachable, No traffic destined for the Network part of the summarized route is going to reach its destination due to the longest match preference.

Could you provide more details about the reason of having 2 Networks spilitted at different sites. I mean why you had to?




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