MORE INFORMATION
In the TCP/IP properties of each of the products listed above, you have the
option to configure a default gateway for each NIC or multiple default
gateways for the same NIC. Every NIC that has a default gateway adds a
0.0.0.0 route (default route) to the routing table.
In either case, the TCP/IP routing table will have multiple default routes
listed (to view the routing table, type "ROUTE PRINT" from the command
prompt). The route most likely to be used is the default gateway for the
primary NIC that is bound to TCP/IP, but this is not always the case. If
the Transmission Control Protocol (TCP) session on the active default
gateway times out, TCP can switch to the next default gateway. This can
present a problem depending on your network configuration.
It is not recommended to have multiple adapters configured on the same
network (it may even create more overhead). For more information on TCP/IP
behavior when multiple adapters are configured on the same network, please
see the following article in the Microsoft Knowledge Base:
175767 Expected Behavior of Multiple Adapters on Same Network
If both NICs are on the same contiguous network, this should not be a
problem as long as the default gateways are valid. However, if the NICs are
configured on disjoint networks, or networks that are physically separated
and cannot communicate with each other, you will not be able to communicate
with the destination host reliably.
A common use of multiple default gateways is to configure a backup gateway
in the event of a failure of the primary gateway (router). This backup
gateway is used by dead gateway detection, and is only triggered with TCP
or connection-oriented traffic. Utilities like PING cannot force the
default gateway to switch, because these utilities use User Datagram
Protocol (UDP) and Internet Control Message Protocol (ICMP).
For additional information, please see the following articles in the
Microsoft Knowledge Base:
128978 Dead Gateway Detection in TCP/IP for Windows NT
171564 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 setting
that must be set for each network adapter. The server is already aware of
all the networks it is directly connected to, and adds a route to each
network for which it has a TCP/IP address.
The default gateway is used only for traffic that needs to go to a network
for which the server has no route. There is only one default gateway active
for a computer at a time. You should generally configure the default
gateway on the most complex network, and leave the field blank on the other
adapter. However, if fault tolerance is desired, choose one of the
following:
- 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:
169161 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.