The function of the crypto is a kmp key cisc406397954 address 0.0.0.0 0.0.0.0 command when establishing an IPsec VPN tunnel is to configure the pre-shared authentication key. This command specifies the key that will be used to authenticate the Internet Key Exchange (IKE) phase 1 negotiation between the IPsec peers. The key is associated with the address 0.0.0.0 0.0.0.0, which means that it will apply to any peer that initiates or responds to the IKE negotiation. This is a common configuration for dynamic IPsec VPN scenarios, such as Dynamic Multipoint VPN (DMVPN) or Easy VPN, where the IP addresses of the peers are not known in advance. However, this is also a less secure configuration, as it exposes the VPN server to potential brute-force attacks from any source. A more secure configuration would be to specify the exact IP address or subnet of the peer, or to use certificates instead of pre-shared keys.
References:
* Implementing and Operating Cisco Security Core Technologies (SCOR) v1.0, Module 4: Securing the Cloud, Lesson 2: Site-to-Site VPNs, Topic: IPsec VPN Configuration
* Cisco IOS Security Configuration Guide: Securing User Services, Release 12.4 - Configuring Internet Key Exchange for IPsec VPNs [Support] - Cisco, Configuring IKE Policies, Step 3: crypto isakmp key keystring [address | hostname] [mask | no-xauth] [netmask mask]