A: While data traffic is blocked on the secondary link, Layer 2 control traffic is still permitted. For example, an LLDP session can be run between two switches on the secondary link.
B: The redundant trunk group is configured on the access switch and contains two links: a primary or active link, and a secondary link. If the active link fails, the secondary link automatically starts forwarding data traffic without waiting for normal spanning-tree protocol convergence.
http://www.juniper.net/documentation/en_US/junos13.2/topics/concept/cfm-redundant-trunk- groups-understanding.html

A: The fourth LSA type, network example:

In this example we have R1 that is redistributing information from the RIP router into OSPF. This makes R1 an ASBR (Autonomous System Border Router). What happens is that R1 will flip a bit in the router LSA to identify itself as an ASBR. When R2 who is an ABR receives this router LSA it will create a type 4 summary ASBR LSA and flood it into area 0. This LSA will also be flooded in all other areas and is required so all OSPF routers know where to find the ASBR.
B: Example:

Router 2 can create a Type 3 summary LSA and flood it into area 0. This LSA will flood into all the other areas of our OSPF network. This way all the routers in other areas will know about the prefixes from other areas. Note: The name "summary" LSA is very misleading. By default OSPF is not going to summarize anything for you. There is however a command that let you summarize inter-area routes. Take a look at my OSPF summarization tutorial if you are interested. If you are looking at the routing table of an OSPF router and see some O IA entries you are looking at LSA type 3 summary LSAs. Those are your inter-area prefixes!