The Gotchas
When I set up dual Internet connections in my office, I encountered a couple of hitches you should be aware of in advance. One involves choosing the right mode for your modem; the other involves a potential difference in Maximum Transmission Units (MTUs) that the router and the DSL connection support. Other concerns, such as managing alerts and updating firmware, were minor.
Setting up pass-through mode. Some DSL and cable modems can operate in either Network Address Translation (NAT) or pass-through mode. In NAT mode, the broadband modem assumes the IP address the ISP supplies, then, through DHCP, assigns your router an address from a 192.168.*.* range. The modem serves as a NAT server, translating each packet's IP address and port numbers as the packet crosses from the LAN to the Internet or vice versa. NAT works fine for simple outbound access. However, to permit incoming connections to servers on your LAN, you don't want your router sitting behind a NAT server. In addition to opening up necessary ports on your router, you would need to open them on your modem as well. Packets would have to cross two NAT boundaries. Therefore, pass-through mode offers a better way to let Internet clients access servers on your LAN.
In pass-through mode, the modem essentially becomes invisible and simply passes packets back and forth, much as a switch does. Your router communicates directly with the ISP's router. Either your router is assigned an address through DHCP or you manually configure your router with the static address that your ISP supplies. If you have a static IP address from your ISP and you want to permit certain incoming connections (e.g., for your email server), make sure your modem is running in pass-through mode.
To find out whether your modem is running in pass-through mode, connect a PC to the modem before you connect your router to the modem. Configure the PC's network connection to use DHCP. If the PC fails to obtain an IP address and configures a self-assigned address instead, your modem is running in pass-through mode. Your modem is also running in pass-through mode if the PC obtains an address that matches the static address that your ISP provided.
If the modem configures the PC with a local subnet address (e.g., 192.168.*.*), it's functioning as a NAT server. To change the mode, access the modem's Web-based administration pages. Open your Web browser and browse to the address of your network connection's default gateway, which is your modem's current LAN-side address. Find the modem configuration setting that lets you enable pass-through mode (aka demilitarized zone—DMZ—server mode). Some modems let you specify a target PC as the endpoint for incoming packets (the target would be the currently connected PC) or target a manually configured device. Select the manually configured device so that the modem will cooperate with your router (configured with the static IP address) when you connect it.
Troubleshooting the DSL's MTU. The other snag I encountered was at first more difficult to pinpoint. Immediately after I set up the dual-router and ISP connections and started testing (by accessing various Web sites), I experienced intermittent connectivity problems in which Web pages would sometimes fail to fully load. The problems increased when I disconnected my cable modem and relied on DSL alone but disappeared when I disconnected DSL. This pattern pointed to a problem with the DSL connection. I discovered that I could consistently access Web pages with little text and few or small images. However, downloads of larger, more image-laden pages would fail after displaying a fraction of the page's content. Finally, I discovered through a discussion forum that BellSouth, my DSL provider, supports an MTU of 1492 bytes, whereas the router defaulted to 1500 bytes. Packets with more than 1492 bytes were usually dropped. When I modified the router's MTU for the DSL connection, the problem disappeared.
Managing alerts. Some dual routers, including the XC-DPG502, alert you when they detect a problem (e.g., an ISP connection going down). At first, I configured my XC-DPG502 to send such alerts to my mobile phone's text-message email address. I received several disconnection alerts every day for hiccups or delays involving one or the other ISP connection. However, thanks to the XC-DPG502's automatic rerouting, none of my users ever reported a problem. Therefore, I chose to turn off the alert feature. If a connection remains down for long, I know about it because my Internet-based monitoring service checks the health of a server behind the router every few minutes.
Updating firmware. Make sure you have the vendor's current firmware installed on your device. Vendor Web sites often have a more-recent version of firmware available than the version preinstalled on the device. Installing the most-recent firmware on my router solved a connection problem I experienced in the first week of use. Having the current firmware also helps you stay patched against known router security holes. Subscribe to the vendor's update list.
Also, be aware that routers usually have a remote administration option that lets you log on to the router's administration Web pages over the Internet. Most routers disable this feature by default, and I recommend you keep it disabled.
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