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Because, by doing manual route summarization you can come up with a much better summary route.
For Example if your Network has (170.12.1.0,170.12.2.0,170.12.3.0) /24 Networks.
EIGRP will automatically summarize to its classfull boundary like170.12.0.0 /16, So you cannot add any routes anywhere in the network starting with170.12. since the router claims all routes starting with170.12.0.0 /16 is through that interface. Hence Manual Summarization.
ip summary-address eigrp90 170.12.0.0 255.255.252.0
disabling auto summary is also a good idea when using EIGRP
R1[C]#Router eigrp [1-155]
no auto-sammary
to reduces the size of routing tables in a network
If a single route goes down that is contained within a summary route, updates are not sent throughout the entire routed domain. Only the router advertising the summary route will know that the more specific route has went down. For EIGRP, this will prevent unwanted queries and potentially SIA in the EIGRP autonomous system.
auto summarisation summarise to classfull address border, however the ip summary-address allow you to adjust you summurasation, that means gives you the ability to costum and then to have a most accurate summarisation
Make better use of network subnets and gurantee for use only ranges we need to summrize
