Pre-shares in hub and spoke
If you are implementing a hub and spoke VPN network and you're like most people, you are probably going to use IKE's "main mode" with pre-shared keys because it is simple and easy. If you already have a hub and spoke VPN network using pre-shared keys and your network has grown, you may have noticed it becoming difficult to manage the keys at your hub router or concentrator. In either case, you should consider changing your IKE mode from "main" to "aggressive".
Both of these modes were specified in RFC2409, which defined the Internet Key Exchange (IKE) and most people are using the main mode, but one of the perks of the aggressive mode is that it supports using RADIUS attributes. RFC 2868, "RADIUS Attributes for Tunnel Protocol Support" specifies a number of options that enhance RADIUS support for various types of VPNs. Two of these attributes are number 66 "Tunnel-Client-Endpoint" and number 69 "Tunnel-Password".
What this allows you to do, assuming your vendor supports this functionality, is to specify your pre-shared keys in your RADIUS profiles instead of in your ISAKMP configuration on your hub device. Of course, if you don't already have a RADIUS server, you'll need to get one.
The advantage here is that it is far easier to manage a large number of keys as attributes of a user profile in most RADIUS implementations than it is to manage the keys in routers or VPN concentrators.
Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.