summarizing eigrp on an interface

Unanswered Question
Aug 6th, 2008

At what point does route summarization on an interface help me? At my host site, I am considering running Layer 3 all the way to the access switches. Best practices talks about summarizing at the distribution layer interfaces. Is this really worth all the work? Does anybody really do this?

I have this problem too.
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Jon Marshall Wed, 08/06/2008 - 10:42

Yes it is worth the work IMHO and it isn't all that much work anyway :-).

We have used a routed L3 access layer and we have made the access-layer switches EIGRP stub and from the distribution layer we advertise a default-route. You can either

1) use a distribute-list at the distribution layer and only allow the routes through to the access-layer that you want - this is what we did with the default-route.

2) Summarise on the L3 interface connection to the access-layer from the distribution layer as you are suggesting.

Either will work fine and summarizing to the access-layer has the same benefits as summarization elsewhere eg. smaller routing tables, easier troubleshooting, a change in one of the subnets in the distribution layer does not cause a recalculation in the access-layer.


DARYLE DIANIS Wed, 08/06/2008 - 10:54

thanks, I'm trying to get an idea of what others do, since I have limited exposure. It seems like the summarization process falls apart with the routed up links, since those are probably a /30 address. I'll have to play around with some numbers.

Jon Marshall Wed, 08/06/2008 - 10:58

Make sure your vlan subnet ranges ie. the user vlans whether data or voice are summarisable from the access-layer.

Don't worry too much about the P2P links used for the uplinks - these could be used from the same class C and advertised out as a class C from the distribution switches.

Edit - if you have multiple sites you would want to ensure that each site had a class C for P2P links ie. don't spread the class C among sites - this really would mess up summarisation.


Richard Burts Wed, 08/06/2008 - 11:12


There is another aspect of this that should be considered. The importance of summarization varies somewhat depending on the number of routes in your routing table. If your entire routing table is perhaps 100 routes then doing summarization may not buy you much. But I have a customer where they have 4000 routes in the routing table. In that case doing summarization has a lot more benefit.

So where does your network fit: few routes or lots of routes? Answering that will help show how motivated you should be to do summarization.



DARYLE DIANIS Wed, 08/06/2008 - 19:51

thanks everyone for sharing your ideas, they are all good. Even though my wan is 20 sites and the lan I am converting to layer 3 has about 6 closets with a 6500 in each for access switches and 2 more 6500's at the distribution layer, I think I will configure everything with summarization in mind. I have one chance to do this.


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