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EIGRP default route and summary

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Last Modified: 2012-05-09
In the BSCI Certification Guide they have the following config.

interface s1/0
ip summary-address eigrp 100 172.xx.x.0 255.255.224.0
interface s1/1
ip summary-address eigrp 100 0.0.0.0 0.0.0.0

the ip summary-address is to summarize  the networks 172.xx.x.0/24 - 172.xx.xx.0/24 back towards the core of the network right?

The guide says "Thus the division is summarized to the larger network and receives back a simple default route."

What does that mean? I can understand the summary address being placed in the routing table and matching any destination in  172.xx.x.0 /19 but how does the default route get matched? Does 0.0.0.0 get installed in the local route table or advertised to other routers?

Sorry it does not have a diagram and it is just confusing me.

It also states: "Summarizing the default route can be a useful technique. However, if used in cases where there are multiple default paths, this will reduce routing options and prevent fail over to alternate paths when the primary path fails."

I don't understand that statement either. We don't have EIGRP at work and I am trying to learn it.
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