Hello @network_geek ,
>> Question 1): Is OSPF virtual-link two way communication or one way?
An OSPF Virtual Link is NOT a form of tunneling that carries user data, it exists only in the control plane / routing plane and it can be seen as point to point unnumbered link that allows to reach area 0 via a STANDARD area making possible the exchange of LSA data structures.
>> Question 2): Suppose a transit link in the path of virtual-link goes down(link between R9 and R10), how does OSPF virtual-link behave? Will the summary LSA generated ever will get removed from LSDB? How will the routing table be effected?
the OSPF virtual link exists until there is an alternate path between OSPF RIDs of the two devices where you have configured it.
In the case of your topology if the link between R9 and R10 fails there is no alternate path within area 10 that interconnects the two routers the ABR( 0, 10 ) and the "ABR" between area 10 "ABR" (10,100) and area 100.
Final note: virtual links are not used in production networks, there are other methods that can be used they are a lab tricky subject.
If you make tests without advertising in OSPF the OSPF RIDs the Virtual link is able to go up.
I had found strange this fact and I asked this in a Ask the Expert session to Vinit Jain a Cisco SME expert and he answered back that the RID even if not advertised can be used as key to seach in OSPF DB the two router LSAs and the nearest interfaces active in OSPF are used to build the control plane only virtual link communication.
Hope to help
Giuseppe
