Install the router. When your network isn't busy, remove your old router and install the dual router. Log on to the router from a PC connected to the internal LAN and recycle your DHCP lease. (You can either run the command
ipconfig /release all
followed by
ipconfig /renew
or simply disable and re-enable your network connection.) After you have a valid IP address with the default gateway pointing to the new router, log on to the router through a Web browser. Verify that the router connects to your current ISP. You might need to reset the cable or DSL modem to help establish connectivity.
After you verify that the router is functioning on the internal network and is reporting a successful connection to your ISP, open another browser and try to access a few Web sites. If access succeeds, you've replaced your router. If it fails, review your configuration settings. If you must change settings, follow sound diagnostic techniques, making one change at a time and testing the result so that you know which change solved the problem. Record each change so that you can reverse settings that aren't required.
Connect the router. At this point, you're still dependent on one broadband Internet connection. To change to two connections, find out whether the new, second ISP assigns addresses dynamically or uses a static address. If the ISP assigns addresses dynamically, connect your new router's second WAN port to the new ISP cable or DSL modem and browse to your router's status page. In a few moments, the router should pick up an address and report that WAN 2 is also connected. If necessary, configure the WAN 2 port with its static IP address, default gateway, and DNS server as your ISP specifies. After the router reports that WAN 2 is connected to ISP 2, it's time to test your fault tolerance.
Disconnect WAN 1 and immediately try to access another Web site. The Web site should come up as usual as the dual router automatically switches to WAN 2. You should be able to switch back and forth with little if any interruption in Internet access. Figure 4 shows the status page of my XCDPG502 with both ISP connections functioning properly.
Added Value
Congratulations. You've just achieved fault-tolerant Internet access for the cost of a second broadband subscription. But this setup offers more than cost savings. Much of the time, both cable and DSL will be up, so you can make the most of them. Most dual-WAN routers support load balancing to split sessions between the two connections. This approach preserves speedy Internet access even during peak usage times. Slightly higher-priced dual routers include site-to-site VPN connections that let you create a virtual WAN between offices. Some routers even let you set up multilink VPN connections that aggregate your combined broadband connections into a single, extra-fast connection between the two sites.
Incoming Connection Failover
If you need to support incoming connections to servers on your WAN (e.g., Web servers, email servers), some routers can act as your Internet-facing DNS server and perform automatic failover from a failed ISP connection to the backup ISP connection. When the router detects that the ISP connection you usually use for incoming email or HTTP requests has failed, it updates the DNS records for those servers with the IP address of your other ISP connection and begins accepting requests on that connection instead. Neither your internal nor Internet-side clients are affected.
To make incoming-connection failover work, you need a static IP address for each connection from its ISP. Static addresses are required because your dual router is serving as the DNS server for your Internet domain name. Although you can change the IP addresses of servers in your Internet domain name and have the changes take effect almost immediately, changes to addresses on your domain name's DNS servers take effect more slowly. Because DNS server addresses are published on the Internet, updates can take up to several days. You need to know the permanent addresses of your DNS servers ahead of time so that Internet clients can still use your alternate DNS server address to resolve the DNS names of your servers.
Also, you need to update your domain's record with your DNS registrar and configure the connections' addresses as the DNS servers of record for your domain. By functioning as the DNS server for your Internet domain, your router can dynamically change the address with which it replies to DNS queries and thereby direct Internet clients to your email or HTTP servers on the ISP connection that's currently up. When a client attempts to resolve your server's DNS name on the Internet and your main ISP connection is down, the client will time out and automatically query the next DNS server listed for your domain—your dual router on the other ISP connection.
Keep in mind, of course, that by relying on one device to provide access to your dual pathways to the Internet, you create a single point of failure. Higher-end dual routers support failover "mates" to eliminate this potential problem. However, in my experience, because solid-state devices (e.g., routers) are more reliable than broadband Internet connections, most SMBs won't need to invest in a backup router.
