Dynamic Routing Using Cisco Routers and GRE Interfaces Over VPNs
One of the best aspects of VPNs has always been their flexibility. Companies can build entire WANs using nothing but VPNs or a combination of VPNs and point-to-point links. One recent trend seen in larger businesses has been to setup high-speed, low cost cable modem or DSL lines with VPNs as a backup link to their point-to-point connections. GRE tunnels become invaluable for dynamic routing should primary links fails. Other companies are choosing to drop their expensive point-to-point links altogether in favor of two (or more) DSL or cable links provided by more than one ISP. Using GRE tunnels, companies can build out their WANs with multiple paths to their data or other offices.