Exam 400-007 topic 1 question 241 discussion

LFA is not correct because it relies on something like BFD or fast hello to detect dead neighbor. The question is a little misleading, but still fast hello will detect dead neighbor under a second in most cases.

I think ans is A. OSPF's Fast Hello feature allows for rapid discovery of adjacent router status by reducing the interval between Hello packet transmissions. This reduces the detection time of dead neighbors to the sub-second level and improves network convergence speed; it is a suitable solution for rapid detection without using BFD.

A - In the absence of BFD, you would select fast hellos, simple. The sub second dead timer requirement is unachievable with the options available, for which only A is valid. LFA is for re-convergence after a loss of route, not to use instead of BFD or fast hellos.

Hello interval can be set at subsecond, but dead interval is one second minimum. https://fredrikjj.wordpress.com/2014/05/17/ospf-timers-part-5/

Answer is D, guys focus on in a subsecond matter which is not supported by fast hello as the dead interval can't be changed to less than 1 sec, while LFA can change to alternative path within 50ms.

fast hello

A is the correct answer - LFA more on the backup route

A is right

 Loop Free Alternates (LFA) is a mechanism by which a neighbor can be used as a backup next hop to provide a local repair path for the traffic to flow temporarily in case of failures in the primary next hop (node or link).  DPD is a method used by devices to verify the current existence and availability of IPsec peers.  The OSPF Support for Fast Hello Packets feature provides a way to configure the sending of hello packets in intervals less than 1 second (sub second). Such a configuration results in faster convergence in an Open Shortest Path First (OSPF) network.