Years ago I’ve been involved in an interesting discussion focusing on NTP authentication and whether you can actually implement it reliably on Cisco IOS. What I got out of it (apart from a working example) was the feeling that NTP and it’s implementation in Cisco IOS was under-understood and under-documented, so I wrote an article about it. Of course the web version got lost in the mists of time but I keep my archives handy.
Last weekend I migrated that article to blog.ipSpace.net. I hope you’ll still find it useful; while it’s pretty old, the fundamentals haven’t changed in the meantime.