There are 2 modes in ISAKMP. Main mode and Aggressive mode. Main mode is slower (more messages are exchanged) than the aggressive mode, but more secure. Aggressive mode on the other hand, is faster (less messages exchanged), but is less secure. You might propably know this. I found Sadat Malik's book (Cisco Press - Network Security Principles and Practices) to have a very good explaination on this topic. All the messages exchanged during both the modes are explained in detail.
