ISP failover can be achieved in two ways. An antiquated method is an ISP-level technique based on the Border Gateway Protocol (BGP). This approach requires a high degree of cooperation among multiple ISPs along with the installation and maintenance of expensive and specialized routers at both ends of a link. Another drawback of BGP is the time it requires to reroute Internet traffic, which can result in costly time lost to Internet delays.

The second approach to ISP failover is a far more economical and reliable business-based solution. This approach uses specialized WAN Optimization Controllers (WOC) located between ordinary routers on a business LAN and the WAN port of the firewall. Each WOC has two or more ports (up to 15 depending on the WOC) to connect to multiple ISPs, and requires no special configuration in the ISPs' routers. When a session is generated from the LAN, the device computes which ISP link has the most available bandwidth and routes the session accordingly. If an ISP link becomes congested, the device automatically reduces traffic going to that link and redirects traffic to ISP links with more available bandwidth. If a link fails, the device automatically stops traffic to the link and redirects it to functioning links.

Inbound ISP failover is achieved by designating the device as the primary and secondary authoritative DNS name server for all the domains being hosted. If an ISP link fails, the device stops advertising that link's IP address to Internet DNS caching servers, which in turn drop that address from their records and redirect traffic to active links. By setting the host name record "Time-to-Live" to a few seconds, the failed link is quickly removed, and is reinstated automatically when link connectivity is restored.

The same technique can be used to provide site failover for business continuity and disaster recovery when a WOC is installed at the primary and backup site. In this approach, the WOC at the backup site continually tests DNS resolution to the WOC at the primary site. If the WOC at the primary site does not respond, the WOC at the backup site immediately initiates the inbound ISP failover procedure as described above. Inbound user traffic is then immediately redirected to the backup site, and Internet-based business operations continue as normal.

FAQ terms to describe WAN optimization