I have question about EIGRP auto summary.
Lets say R1 is connected to R2 and R2 is connected to R3.
R2 and R3 have auto summ on by default.
R1 has no auto summary configured.
If R1 advertise about its Lan network 10.10.10.0/24
Router EIGRP 100
no auto summary
When router 10.10.10.0 reaches R3 will it be shown as or 10.0.0.0/8 classfull address in R3s routing table?
Here is the straight forward concept for Eigrp Auto-Summary that you wouldnt forget in the future:
Auto-summary: When a router running Eigrp have Auto-summary enabled, this router summarizes all network updates between (DIFFERENT MAJOR NETWORK BOUNDARIES).
This means, if the major network between different routers running eigrp with auto-summary enabled are the SAME, then it WILL NOT summarize any network updates.
Lets take a look at your example:
R1 has no auto-summary and advertises Network 10.0.0.0/24 to R2, if the Link between R2 and R3 is from the same major Network 10.x , for example 10.1.1.1/24, then R2 WILL NOT advertise a summary route of 10.0.0.0/8 to R3, Even though R2 has Auto-Summary enabled. (because they are not in DIFFERENT Major Network Boundaries).
However, if the Major Network between R2 and R3 is different than 10.x, then it will advertise a summary route of 10.0.0.0/8 to R3. for instance, if the Link between R2 & R3 is from the Network 192.168.x or 172.16 or Whatever but NOT the 10.x, then a Summary of 10.0.0.0/8 would be sent from R2 to R3 , and R3 would have 10.0.0.0/8 in its routing table.
I hope this concept is clear for you and let us know if you have further inquiries,
To answer your question directly I might say that R3 would have a route for 10.10.10.0 with a /24 prefix-length and not /8 even if R3 and R2 are configured with auto-summary as default.
A little explanation about the auto-summary feature : if a router has at least 2 interfaces in different classfull network, the default behavior is to advertise a classfull summary for these two networks (if matched by a network command...).
Auto-summary feature has no effect on route being learned by a router but only when the router sends an update.