IKE is a key management protocol designed for the secure exchange of encryption keys used by other encryption and authentication schemes. 3DES and SHA-1 provide higher levels of security, but using 3DES may slow VPN response time because it requires a lot of processing power to encrypt and decrypt data. IKE is the framework that negotiates security association and keys, allowing each end of the connection to authenticate itself to the other and establish shared keys. Within IKE, two modes are used. Main mode provides identity protection for the hosts initiating the IPSec session but takes slightly longer to complete.
Aggressive mode provides no identity protection but is quicker.
The mode you choose to use largely depends on the configuration of the other VPN devices. For the communication to occur successfully, both systems must be configured to use the same IKE mode. Some remote access products only support aggressive mode. With all these modes, parameters, and negotiations to consider, you can see that establishing a VPN tunnel is not a straightforward process. Every one of these configuration details must match on each end for the negotiation to occur successfully.