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 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 since that’s the classful network.


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

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

Now since auto-summary is still turned on, router R1 will summarize the network to the classful network of


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!