Configuring a Network Bridge
If you install a wireless card into a laptop or a Phoneline segment in your office, you will want to connect these devices into your main network. Figure 20.39 shows an example.
Figure 20.39. Office with a mix of physical network devices bridged at a server running Windows Server 2003.
With Windows Server 2003, Standard Edition, or XP desktop, you have the option of either routing or bridging between the segments. The problem with routing in a SOHO network is that you need to configure multiple IP subnets. This can get a little complicated in what is supposed to be a simple networking environment. For simplicity, bridging is a better solution.
Both Windows Server 2003, Standard Edition, and XP have a Network Bridge feature that can be used in place of routing to connect interfaces in different network segments. (Bridging is not available in the Enterprise or Datacenter Edition packages.) Windows treats a Network Bridge as a distinct device with its own interface and IP address. This eliminates the complexity of running a multihomed server.
An XP desktop is capable of routing, also called IP forwarding, but the feature is not exposed in the User Interface (UI). You can make a Registry change to enable the feature. Here is the setting:
Key: HKLM | System | CurrentControlSet | Services | Tcpip |
Data: 1 (REG_DWORD)
As I'm sure you know, bridging connects interfaces at the data link layer of the OSI networking model. At this layer, only the physical MAC addresses assigned to the interfaces matter. Bridging uses a spanning tree algorithm to sort out where the frames should be sent.
A spanning tree is essentially a big lookup table. The bridge service builds the spanning tree table by looking at the MAC address of any frames that arrive at each interface. It makes a little note that says, "Device 00-b3-a9-37-ef-1a came in on the Phoneline adapter." If, at some later time, a frame destined for that MAC address arrives on another segment, the bridge shuttles the frame onto the interface for that segment and sends it on its way.
The disadvantage of bridging over routing is its limited scalability. MAC address lookups are not a particularly sophisticated way to shovel traffic around a network. A spanning tree table does not do a good job of handling segments with large numbers of hosts. You have to be careful about accidentally creating spanning tree loops by interconnecting segments in two different places. Also, bridges do not block broadcasts or multicasts.
Still, for a small office where only a few users have alternate media devices, using a bridge to connect disparate network segments works quite well.
Configuring a Network Bridge
A Network Bridge is configured in the Network Connections window. There is really only one step to the process. You need at least two network adapters in the server.
Hold the Control key down and select the interfaces you want to bridge together. Then, right-click and select BRIDGE CONNECTIONS from the flyout menu. The system thinks about this for a while then creates a new icon that represents the bridge. Figure 20.40 shows the results.
Figure 20.40. Network Connections window showing a Network Bridge.
After the bridge is up, the underlying connections are abstracted behind it. Utilities such as IPCONFIG only show the bridge, not the actual adapters. This can take a little getting used to. Still, after the bridge is in place, you can pretty much forget it is there until you have to do maintenance on the server.
If you have several LAN segments in your office, be especially careful not to bridge in more than one place unless you map our your topology carefully. If you create a loop, the spanning tree will crash and all interconnections will cease.