Anyone can explain what is "ip summary-address eigrp" used for on an interfaces?? What is the scenario likely to be?
It is used to advertise a summary route down the link. An example might help.
Lets say you have an access-layer switch on a floor connecting to a distribution switch. Lets also say that the link between the access-layer switch and the distribution switch is a L3 link so the distribution and access-layer switch are EIGRP neighbours.
In the access-layer you have the following vlans
Rather than adverise out each individual network you can just advertise a summary route 192.168.1.0 188.8.131.52 using the "ip summary-address eigrp" command. You would configure this under the interface configuration on the access-layer switch that connects to the distribution switch.
1) Smaller routing tables used less resources
2) Summary routes "hide" individual link flaps ie. if one of your vlan interfaces goes down this does not have to be propogated throughout your EIGRP domain.