Hi,
Here is the concept for both (Auto Summarization & Summary Address) in Eigrp:
1- Auto Summary:
When (Auto Summary) is enabled, Eigrp performs summarization between major Network boundaries, lets assume you have 2 Networks 10.10.10.0/24 and 172.16.1.0/24, and you have enabled auto summarization in eigrp. all Network routing updates from the 172 segment towards Network 10 will be summarized to its major class which is 172.16.0.0/16 (since its class B) and all Network routing updates from 10 towards the 172 Boundary will be summarized to its major class which is 10.0.0.0/8 (since its class A).
2- Summary Address:
When a summary address is configured per interface in Eigrp for an exist componenet subnets, a summary route pointing to Null0 interface is created in the routing table. This summary address is used to prevent loops in Eigrp. However the Summary is not sourced from the Null0 , The summary is generated for the components subnets as advertised to any Eigrp neighbor configured with a summary address.
HTH
Mohamed