Basic EIGRP Auto-Summary

In the above topology I have auto-summary enabled on R1 and R2. R3 does not have auto-summary. The result of this config is R1 and R2 will actually advertise the 10.0.0.0/8 network, since both of those 10. networks reside in that classful boundary.  Since their only neighbor is R3, R3 will have a duplicate route for 10.0.0.0/8 since that’s the classful network.

 

Now R3 thinks it has two equal cost paths to the 10.0.0.0/8 network, which is clearly wrong.

Just to make it more clear I will now add the 172.16.1.0/24 network to R1.

Now since auto-summary is still turned on, router R1 will summarize the 172.16.1.0/24 network to the classful network of 172.16.0.0/16.

 

Now let’s fix the issue.

Notice how the neighbor is soft-resync’d. Now on R3 we should see the real network advertisements.

 

Perfect now our routes are being advertised as they should be!