Hello Narad
Recursion is the process that an OSPF router takes to “solve” the shortest path algorithm. When a type 5 LSA is received, it must be “recursed” to the point where there is a type 1 LSA in the LSDB that points to the next hop router.
For example, take a look at this image:
R2 receives a type 5 LSA from R1 which is an intra-area ASBR, you will see the type 5 entry in the LSDB, but that entry will have a forward address of 0.0.0.0. This means that in order to reach the destination, the router must solve the shortest path to the ASBR. You will also see in the LSDB that there is a type 1 LSA that points to R1 which is the intra area ASBR. So R2 recurses the type 5 LSA to a type 1 LSA and solves the shortest path to reach that external prefix.
A similar process takes place with a type 3 LSA as well as a type 5 LSA that is received from an inter-area ASBR as well.
The routing bit is an internal implementation variable that is used only by Cisco. It is a bit that is set on an LSA only if the LSA should be considered during the SPF calculation. The bit is set only after the LSA has passed all necessary checks. This makes it more efficient so that these checks don’t have to be performed over and over. If the routing bit is set, the router knows that the LSA is eligible for processing. Note that this is a bit in the options field of the LSA and is not propagated from router to router.
Take a look at this post:
As stated in this Cisco documentation:
Data traffic for the advertised destination will be forwarded to this address. If the forwarding address is set to 0.0.0.0, data traffic will be forwarded instead to the advertisement’s originator.
You can find detailed information about this in the following lesson:
I hope this has been helpful!
Laz