There is an implicit deny all at the end of any route-map so all other traffic that does not match 172.20.5.0/24 would be dropped. Therefore we have to add a permitsequence at the end of the route-map to allow other traffic.
The default value of Local Preference is 100 and higher value is preferred so we have to set the local preference of AS100 lower than that of AS200.

Explanation
By using "deny" keyword in a route-map, we can filter out the prefix specified in the prefix-list.
But there is an implicit "deny all" statement in the prefix-list so we must permit other prefixes with "permit" keyword in the route-map.

Explanation
If you configure the redistribution of OSPF into BGP without keywords, only OSPF intra-area and inter-area routes are redistributed into BGP, by default. You can use the internal keyword along with the redistribute command under router bgp to redistribute OSPF intra- and inter-area routes.
Use the external keyword along with the redistribute command under router bgp to redistribute OSPF external routes into BGP.
-> In order to redistribute all OSPF routes into BGP, we must use both internal and external keywords. The full command would be (suppose we are using OSPF 1):
redistribute ospf 1 match internal external
Note: The configuration shows match internal external 1 external 2. This is normal because OSPF automatically appends "external 1 external 2" in the configuration. In other words, keyword external = external 1 external 2. External 1 = O E1 and External 2 = O E2.
Reference:
https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/5242-bgp-ospf-redi s.html

Explanation
From the exhibit, we learned:
+ VRF customer_a was exported with Route target (RT) of 1:1 so at the remote site it must be imported with the same RT 1:1.
+ VRF customer_a was imported with Route target (RT) of 1:1 so at the remote site it must be exported with the same RT 1:1.
Therefore at the remote site we must configure the command "route-target both 1:1" (which is equivalent to two commands "route-target import 1:1" & "route-target export 1:1".