Ipsec Site to Site VPN Guide
Site to Site VPN
Site to site VPN is a VPN tunnel between two or more sites. This would allow offices to share files and other resources. A VPN tunnel would be created using VPN gateways on each site usually using IPSec to secure the VPN connection over the internet.
When a tunnel has been created between sites, users are able to access and share files and resources easily. However this would all rely on an internet connection and relying that both sites ISP's are up. Some site to site VPN's are configured using multi-wan setup which would provide them with some redundancy if an ISP went down. So on their VPN they would have two ISP's connected. The primary ISP would usually be the faster internet connection, and they would have a slower link connected as a back-up link. This backup link would come into affect if the primary ISP goes down.
VPN's can also be setup in a site to multi site configuration. So you would have all branch offices connected to the head office VPN. The branch offices can connect to each other via the head office. This is usually referred to as a hub and spoke deployment. The head office is the hub, and the branch offices are the spokes connecting to the hub. The head office VPN appliance would need to be powerful and scalable to provide connectivity to all branch offices.
Wikipedia's guide to VPN