Hello Peter,

I need some further explanation. As you commented:

"Assume now that R2 suddenly fails. R1 now looks into its topology table, first looking for the next shortest path to R5 - and it will come across the path through R4, with the total metric of 21. However, at the same time, R1 will find out that R4 is not a Feasible Successor. Therefore, R1 will go to Active state, send out Queries, and after receiving Replies from R3 and R4 (both claiming the same distance as before as they are not affected by R2's outage), R1 will be allowed to reset the Feasible Distance and set it to the metric of the new shortest path - which is 21, through R4, with R4 becoming Successor right away and R3 remaining the Feasible Successor."

My understanding of this situation was that if R2 lost then feasible successor will be promoted to successor . Router1 in this situation will not go to ACTIVE. Please correct me if I am wrong.

Now if my understanding is correct then R1-R3-R5 will be next successor route. If this path is also lost then Router1 will go to active and send the query message. My question was , then what is the point of keep non-FS route in the topology if we have to discover the new route through Query message.

Many thanks for your time and support.

Wajih