Suppose each laptop has an Ethernet port and a WLAN PC card, and both are enabled. When the Ethernet cable is plugged into the laptop and the laptop is not within reach of the 802.11 network, XP will use the Ethernet adapter and appropriate network routes. When the Ethernet is unplugged, XP will remove those routes. When the laptop moves within range of the 802.11 network and associates with an access point, XP will update its routing table to start using the wireless adapter.
What happens when the laptop is simultaneously plugged into the Ethernet and within reach of the 802.11 network? When both interfaces are active, XP will have two default routes. Assuming that your Ethernet and 802.11 networks are separate subnets, the laptop will use Ethernet to reach destinations on the wired LAN and 802.11 to reach destinations on the wireless LAN. If the routing metrics are equal, the laptop may try to reach destinations beyond the local LAN or WLAN through either adapter. If both can reach other destinations, perhaps that is fine. If that's not what you want, you have some options:
- Users can manually enabled/disable connections using XP's network connections control panel. For example, when Ethernet is desired for higher throughput, disable the wireless adapter.
- Using the network control panel TCP/IP properties, configure a default gateway metric for the Ethernet adapter so that it is always preferred over the 802.11 adapter's default gateway (assuming that's the behavior you are after.) This is a one-time change you'd need to make on every laptop.
- Use a mobile VPN product that automatically picks the highest-bandwidth adapter (for example, NetMotion.) This may be warranted if you need to sustain application sessions when the laptop's active connection changes. Otherwise, when the laptop's IP address changes, TCP sessions in progress to the old IP address will be disconnected. Whether that's a problem for you depends on what applications your users run. Network-layer VPN tunnels (PPTP, IPsec) also get disconnected when roaming between networks.
This was first published in March 2003