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When A EIGRP router loses a successor route and it does not have in its table a feasible successor route, it will go active, actively looking for another path to that lost network that he lost. In the active state, a EIGRP router send Queries asking the other routers he’s connected to and it’s neighbour with, if those routers doesn’t have a route to that network he just lost. The Query message is sent and a timer is started, this timer is used to prevent the sending router to remove other routers from its table if they do not respond to his Queries and that are waiting themselves for other routers to respond to them for that Query, because they are active for that route as we are.

So, in other words, if I sent you a query asking for way to a specific route that I lost and you do not respond to me back, because you are in the active state and waiting for a reply yourself from other routers for that specific route, I will send you a Stuck In Active Query or shortly SIA Query, and I will wait for 90 seconds. After 90 seconds, I’ll say “Hey neighbour, are you still here ?” and if I don’t get any reply from you, I will remove my route to you from my table, If I do get a reply from you, you’ll tell me “Yes buddy, I’m still here, just waiting for others to respond to your Query”, then I will not remove my route to this neighbour from my table.

Even with this operation there is sometimes a lot of fun going on in the network, because a Stub Router lost a route and he’s Quering the whole network and consumes bandwidth and increases CPU and delay and a potential for a bigger problem than his lost route.

Fortunately, there is a feature called Stub Router in EIGRP which solves this stuff. Imagine that I’m R1 and below me is hanging R2 on a link, on R2 is hanging R3 and R4, on R3 and R4 is hanging R5 and R6 respectively. Logically, I will make my Router R1 a Stub Router, because he is connected just to one neighbour, to R2 and that’s it. This way, when R1 will lose a route from it’s table, he will not send a Query message asking other routers if they do have a path to that route, because it’s useless, maybe that route that R1 have lost is a Link to some end users, or just a loopback interface which we used for testing purposes for other thing and now we shut down this loopback by mistake, this way creating the whole network to flood a queries for that route, which of course they do not have another path to. So, here’s how to make R1 a Stub and limit this Queries messages to be sent from R1 and even from R2, because he will see that I’m a Stub Router and Queries are supressed.

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