I have two questions and need detailed answer on these, so that I can perform this action -
1>If I do not have access to devices, I have the running config of router , firewall and core switch , and I need to prepare the network topology and network diagram and route flow diagram. what should be the approach, what all things to consider to prepare it.
2> If I have access to devices of Router, Firewall and Core switches, what should be the approach and commands to be used to know about the network and using those information to prepare the topology and network diagram.
If you just have the configuration files, you can still determine the network topology and route flow diagram. Looking at the interfaces, the IP addresses and subnets, as well as the routing configuration, you should be able to get most of the info you need. If you want to get a hands-on approach, use GNS3 or another emulator to recreate the topology and see how it behaves.
Now either from the real topology, or your emulated one, looking at additional things such as routing tables, CDP neighbors, and initiating traceroutes will also help you to get a clearer picture of how things are configured, to verify your results.
I hope this has been helpful!