Aksher,
IKE is a protocol used by IPSec for completion of Phase 1. IKE negotiates and assigns SAs for each IPSec peer, which provide a secure channel for the negotiation of the IPSec SAs in Phase 2.
If you are terminating IPSEC tunnels on the outside interface, then you need to enable isakmp on the outside. In case if you are terminating IPSEC Tunnels on the DMZ, then you need to enable Isakmp on the DMZ.
Please refer the below URL for details:
http://www.cisco.com/univercd/cc/td/doc/product/iaabu/pix/pix_sw/v_62/config/ipsecint.htm#wp1028911
Let me know if it helps.
Regards,
Arul
