In the next month my functions will be expanded to include IT security (firewalls, VPN, PKI) in a big complex infrastructure. Can I ask you for advice on how-to be well prepared for it? What sorts of tools and checklists should I use when transferring institutional knowledge from colleagues who have decided to leave the company?
You'll want at a minimum:
1.) logical infrastructure diagrams
2.) physical infrastructure diagrams
3.) usernames/passwords at the admin (enabled) level
4.) peer router lists
5.) trusted user lists
From this point you can audit configuration settings and start baselining the current network activity. Where you go from there is up to you.
Some great information already provided, I would also add: