For inbound network connectivity, PowerLink and ShieldLink use DNS to direct external users to the optimal ISP link. For example, when a user accesses a site, a DNS request is made to PowerLink requesting the sites IP address. PowerLink responds with the sites IP address that directs the user to the appropriate ISP or WAN link for that site. If a poorly-performing or failed link is found, PowerLink immediately switches users over to other functioning links to ensure reliable and fast connectivity. Incoming link load balancing is accomplished by PowerLink acting as the authoritative DNS server for the domain. PowerLink advertises all available WAN lines to the DNS cache servers which in turn resolve the domain names to queries in a round robin format. In this manner, all externally initiated sessions are load balanced over all available links. Since PowerLink is resident at the domain site and is able to directly monitor the link status, failed links are removed from the DNS tables immediately upon failure. By setting the host name record Time-to-Live (TTL) to a short period (i.e. 30 seconds), the DNS caching servers will flush their address tables and will update them from the PowerLink regularly and thus be informed when a link fails.

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