Most probably you are using "ip summary-address" eigrp summarization command. This causes the creation of an EIGRP summary default route to the null 0 interface with an administrative distance of 5, most commonly to prevent routing loops.
One basic working principle of eigrp is that eigrp will advertise a route only when it sees that that ip is one of its active interfaces. For ex. if you advertise a network in eigrp using "network 10.10.10.0 0.0.0.255" command, eigrp will look into the interfaces on the router and check if any of the interfaces are enabled with ip address of 10.10.10.x. Only then it advertises that route. Now, when you advertise a summarized network in eigrp, that summarized network ip will mostly not be resembling any of your active interface ip. Hence EIGRP creates a logical null interface and points that summarized route to the null interface.
The earlier post on this thread was correct--it's to prevent routing loops. We don't create the discard route (the route to null 0) to advertise the summary route. EIGRP will always advertise a summary route, as long as there's some internal component within the summary range, no matter what local interfaces are connected.
Hi everyone, I would like to thank you in advance for any help you can provide a newcomer like myself!
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