A multihomed computer is one that has multiple network interfaces. Forcomputers using TCP/IP, these interfaces can be separate network interfacecards (NICs) or multiple IP addresses on one NIC. This article discussesthe recommended method of configuring the default gateway on multihomedcomputers.
In the TCP/IP properties of each of the products listed above, you have theoption to configure a default gateway for each NIC or multiple defaultgateways for the same NIC. Every NIC that has a default gateway adds a0.0.0.0 route (default route) to the routing table.
In either case, the TCP/IP routing table will have multiple default routeslisted (to view the routing table, type "ROUTE PRINT" from the commandprompt). The route most likely to be used is the default gateway for theprimary NIC that is bound to TCP/IP, but this is not always the case. Ifthe Transmission Control Protocol (TCP) session on the active defaultgateway times out, TCP can switch to the next default gateway. This canpresent a problem depending on your network configuration.
It is not recommended to have multiple adapters configured on the samenetwork (it may even create more overhead). For more information on TCP/IPbehavior when multiple adapters are configured on the same network, pleasesee the following article in the Microsoft Knowledge Base:
Expected Behavior of Multiple Adapters on Same Network
If both NICs are on the same contiguous network, this should not be aproblem as long as the default gateways are valid. However, if the NICs areconfigured on disjoint networks, or networks that are physically separatedand cannot communicate with each other, you will not be able to communicatewith the destination host reliably.
A common use of multiple default gateways is to configure a backup gatewayin the event of a failure of the primary gateway (router). This backupgateway is used by dead gateway detection, and is only triggered with TCPor connection-oriented traffic. Utilities like PING cannot force thedefault gateway to switch, because these utilities use User DatagramProtocol (UDP) and Internet Control Message Protocol (ICMP).
For additional information, please see the following articles in theMicrosoft Knowledge Base:
Dead Gateway Detection in TCP/IP for Windows NT
TCP/IP Dead Gateway Detection Algorithm Updated for Windows NT
Only one default gateway should be configured on any multihomed computer.The default gateway is a global configuration for the server, not a settingthat must be set for each network adapter. The server is already aware ofall the networks it is directly connected to, and adds a route to eachnetwork for which it has a TCP/IP address.
The default gateway is used only for traffic that needs to go to a networkfor which the server has no route. There is only one default gateway activefor a computer at a time. You should generally configure the defaultgateway on the most complex network, and leave the field blank on the otheradapter. However, if fault tolerance is desired, choose one of thefollowing:
- If multiple routers are available on the same subnet, configure one (or more) default gateways on the same network adapter. This allows you to know which one is actually the primary. Leave all other adapters' default gateway configuration blank. -or-
- If the adapters on the multihomed computer are on disjoint networks (subnets that are not connected by a router), configure one (or more) default gateways on the same network adapter and use static routes for the remote networks that are reached through the network adapter with no default gateway. -or-
- Use a Routing Information Protocol (RIP) listener by enabling silent RIP and broadcasting available default routes with unique costs. The preferred default gateway can be the one advertised with the lowest cost. For more information, please see the following article in the Microsoft Knowledge Base:
Registry Parameters for RIP for IP Version 1
- The simplest way to control the ordering of default gateways is to add only a single default gateway through the TCP/IP properties in Control Panel and add any additional gateways using the command line utility Route.exe. By providing a unique cost for each gateway, the ordering of the gateways can be easily determined.