In EIGRP, if Successor route fails and there is no feasible successor route available, how EIGRP will react?
There are sometimes the situation happens that you have described above. If that happens the EIGRP declares the route as SIA " Stuck in Active " routes. When you do " show eigrp active " its lists the router address from which the reply is pending and then you can troubleshoot the issue.There could be some problem with the router, may be some bandwidth issue, some link issue due to which a reply is pending.
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