If for some reason a router enters into the Diffusing Computation to find a new loops free path that meets the FC…if for any reason neighboring routers have to start the diffusing computation on their own…every router in that path is dependent on the adjacent neighbors to reply back before the route can potentially go back to passive.
In some large and complex networks the Query can go deep into the network and the originating router will never get a response back….or if there is degradation in the path preventing Reply’s from being sent back….these can cause the router to remain in the Active state waiting for a Reply….otherwise referred to as Stuck in Active state.
As a result EIGRP has implemented some features to deal with this potential issue. Once a Query packet is sent and the Diffusing computation begins, an Active timer starts. Active timers default to 3 minutes (configurable)…if that time expires and no Reply to the Query has been received, all neighbors to that destination prefix are torn down and considered infinite in metric.
Tearing down a neighbor in early EIGRP implementations introduced other problems such as legitimate routes still being advertised from that neighbor being torn down as a result of the neighbor being taken down…causes issues.
This is where the SIA-Query was introduced, in half the Active timer time the router will send its neighbor a SIA-Query message, if the neighbor receives the packet and is able to reply it will immediately send back an SIA-Reply message.
SIA-Reply simply acknowledges if the neighbor expects a Reply back from further down neighbors or No the computation is finished and provides the current metric to the destination….There is no waiting for these messages, they’re sent immediately one way or another.
If the neighbor is still waiting on a response, this will increase the amount of time allotted for the diffusing computation to complete without tearing the neighbor down.
If two neighbors are unable to communicate the SIA messages between each other it’s likely this is where the problem exists and the neighbor will get dropped as normal.
Be The Packets - CCIE Study Blog 