The Cable Guy - April 2005

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05/05/2010

Windows XP and Windows Server 2003 Behavior When Connected to Both Wired and Wireless Networks

TechNet's The Cable Guy

Introduction

Modern laptop computers come equipped with both Ethernet and wireless network adapters. The Ethernet adapter typically supports both 100BaseT and 10BaseT. The wireless network adapter might support 802.11b (11 megabit per second [Mbps] maximum bit rate), 802.11g (54 Mbps maximum bit rate), 802.11a (54 Mpbs maximum bit rate), or a combination of 802.11 technologies. When a wireless laptop is undocked from its docking station, it has only 802.11 wireless connectivity. However, when the laptop computer is docked, it can have both Ethernet and 802.11 wireless connectivity.

This article describes how computers running Microsoft Windows XP or Windows Server 2003 behave when they have simultaneous connectivity to both wired and wireless networks. This article also describes workarounds for ensuring either that only one network adapter is active at any one time, or that the majority of the network traffic flows over the desired network adapter.

Connecting to only a Single Network

Although simultaneous connectivity can have its benefits as a fault-tolerant way to maintain a connection to an intranet, it can also be viewed as a disadvantage for the following reasons:

Having two network connections uses up two Internet Protocol version 4 (IPv4) addresses. In some networks, Dynamic Host Configuration Protocol (DHCP)-allocated IPv4 addresses are scarce and a client computer should only be using one at a time.
With two connections, it becomes more difficult to determine the connection over which network traffic is flowing.
For some wireless infrastructures, wireless bandwidth is relatively scarce due to a small number of deployed wireless access points (APs). Therefore, wireless connections should only be used when a laptop is roaming and away from its docking station.

At this time, computers running Windows XP or Windows Server 2003 do not have the capability to automatically disable the wireless network adapter when the Ethernet adapter is plugged into a network and to automatically enable the wireless network adapter when the Ethernet adapter is unplugged. If you do not want simultaneous connections to both wired and wireless networks, you must manually enable and disable the connection to the wireless network, the procedure for which varies with different releases of Windows XP and Windows Server 2003.

For computers running Windows XP with no service packs installed, Windows XP with Service Pack 1 (SP1), or Windows Server 2003 with no service packs installed, you can manually enable or disable the wireless network adapter from the notification area of the desktop or from the Network Connections folder. To enable, right-click the wireless adapter icon and then click Enable. To disable, right-click the wireless adapter icon and then click Disable.

For computers running Windows XP with Service Pack 2 (SP2) or Windows Server 2003 with Service Pack 1, you can use the same procedure to manually enable and disable the wireless adapter. You can also use a new feature in Windows XP SP2 and Windows Server 2003 Service Pack 1 to connect to your wireless network manually, rather than automatically.

To manually disconnect from your wireless network, right-click your wireless adapter icon in the notification area, and then click View Available Wireless Networks. From the Choose a wireless network dialog box, double-click your currently connected network. You will be prompted with a message asking whether you want to disconnect from the wireless network. When you select Yes, you will be disconnected. The status of the wireless network in the Choose a wireless network dialog box changes from Connected to Manual, indicating that you must manually specify when you want to connect. To manually reconnect to the wireless network, double-click the name in the Choose a wireless network dialog box. You can also configure the wireless network for an on-demand connection by clearing the Connect when this network is within range check box on the Connection tab for the properties of the wireless network.

Traffic Behavior when Simultaneously Connected to Both Wired and Wireless Networks

The following is a summary of the network traffic behavior when a computer running Windows XP or Windows Server 2003 is connected to both a wired and wireless network (hereafter referred to as a multihomed computer):

For traffic initiated by other computers that are neighbors of the multihomed computer, the traffic flows over the network adapter attached to the common subnet. For example, if a neighboring computer on the Ethernet-based subnet of the multihomed computer initiates traffic, the traffic flows over the Ethernet network adapter.
For traffic initiated by other remote computers (located beyond the locally attached subnets), the traffic flows over the network adapter corresponding to the destination IP address chosen by the initiating computer. For example, a multihomed Windows XP-based computer will register the IP addresses for both wired and wireless network adapters in the Domain Name System (DNS) using DNS dynamic update. When another computer queries for the name of the multihomed computer, it will get both of the multihomed computer's IP addresses in random order. The DNS client resolver in Windows XP chooses the first IP address in the list returned by the DNS server.
For traffic initiated by the multihomed computer for neighboring destinations, the traffic flows over the network adapter attached to the common subnet. For example, if a neighboring computer is on the Ethernet-based subnet of the multihomed computer, the traffic flows over the Ethernet network adapter.
For traffic initiated by the multihomed computer for remote destinations, the traffic flows over the network adapter associated with the currently chosen default route in the IP routing table, unless there are additional routes to the remote destination.

Assuming that the multihomed computer is running mostly client applications and accessing servers on remote subnets, most of the traffic of the multihomed computer is in the last category (traffic initiated by the multihomed computer for remote destinations).

TCP/IP for Windows determines the current default route from the following criteria:

Select the default route that has the lowest metric.
If there are multiple default routes with the lowest metric, choose the default route corresponding to the network adapter that is highest in the binding order. To view and modify the binding order, open the Network Connections folder, click Advanced, and then click Advanced Settings. The adapter binding order is displayed in the Connections area of the Adapters and Bindings tab.

By default, TCP/IP for Windows determines the metric for the default route by using the Automatic Metric feature, which assigns the metric to routes associated with the configuration of an adapter based on its link speed. For more information, see An explanation of the Automatic Metric feature for Internet Protocol routes.

Table 1 lists the combination of wired and wireless technologies, their associated automatic metrics, and the resulting chosen default gateway adapter for a multihomed computer running Windows XP with SP2.

Wired technology	Automatic metric	Wireless technology	Automatic metric	Chosen default gateway adapter
10BaseT	30	802.11b	30	Uses binding order
10BaseT	30	802.11a or 802.11g	25	802.11a or 802.11g (wireless)
100BaseT	20	802.11b	30	100BaseT (wired)
100BaseT	20	802.11a or 802.11g	25	100BaseT (wired)

Table 1. Combinations of wired and wireless technologies and the chosen default gateway adapter

To override the automatically calculated metric for the default route of a manually configured IP address configuration, specify the default route metric from the advanced properties of the Internet Protocol (TCP/IP) component. For more information, see Default Gateway Behavior for Windows TCP/IP.

To override the automatically calculated metric for the default route for DHCP client computers, you can use the Default Router Metric Base Microsoft vendor-specific DHCP option. To change the automatically calculated metric for the default route for all the Windows XP and Windows Server 2003-based DHCP client computers on a specific subnet, add the Microsoft Default Router Metric Base option as a scope option to the DHCP scope corresponding to the subnet. When adding or configuring this option, specify Microsoft Windows 2000 Options as the vendor class and Default User Class as the user class on the Advanced tab. The following figure shows an example.

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To change the automatically calculated metric for the default route for a subset of Windows XP and Windows Server 2003-based DHCP clients on a specific subnet, do the following:

Create a new DHCP user class. For more information about DHCP user classes, see Microsoft DHCP Vendor and User Classes and How to Create a New DHCP User or Vendor Class.
Configure the specific DHCP clients with the same user class with the ipconfig /setclassid command.
Add the Microsoft Default Router Metric Base option as a scope option for the scope corresponding to the subnet on which the specific DHCP clients are located. When adding or configuring this option, specify Microsoft Windows 2000 Options as the vendor class and the new user class as the user class on the Advanced tab. The following figure shows an example.

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For More Information

For more information about wireless support in Windows and default gateway behavior, consult the following resources:

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