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I have 2 NICs in my machine, and we have 2 internets services on our LAN; one is a DSL one is a partial T-1.

I have one NIC with the DSL as the gateway, and the other NIC has the T-1 as the gateway. So, how does Windows XP know or decide which one to use when I go surfing the web?

When I want to force a certain one (becuase of load), I currently handle it manually by disabling the one I do not want to use. That forces it to use the other one.

It all works fine, but I'd like to know more about how determine the which one it will when both are active.

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Are you asking for your PC individually, or are you asking in order to make the whole network perform better? –  tomjedrz Jun 5 '09 at 13:35
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4 Answers

Open up Network Connections and choose Advanced Settings from the Advanced menu. This will show you / allow you to config the ordering of NICs, service bindings for each NIC and service proivders.

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Not to take this another route (pun)...but have you considered a single NIC in your machine pointing to a managed switch or firewall that is connected to both the DSL line and the T1?

This way you eliminate the routing on the client workstation and let your networking equipment handle the default route(s) and failovers.

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I agree with this. –  Mike Conigliaro Jun 5 '09 at 13:44
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The simplest way to figure out what interface your traffic is going through is to look at your route table.

Linux: netstat -r
Windows: route print

Alternatively, you can use a (win32)tracert/(linux)traceroute/mtr to see the path you are taking to get to your destination.

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Actually if both NICs have a default gateway, you will have two 0.0.0.0 routes. The binding order is what matters. –  Jim March Jun 5 '09 at 13:38
    
If there are no other networks beyond the subnet you are connected to on the NIC you do not want to use for Internet, you can also just remove the default gateway from that NIC. It will only route to the connected subnet. –  Kevin Kuphal Jun 5 '09 at 14:26
    
Jim: In my experience w/unix whatever interface is setup as your default gateway, you are gauranteed all traffic will go out that interface, unless you have added routes. As for windows, yes they allow you to prefer interfaces over one another. –  buecking Jun 5 '09 at 14:50
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It all works fine, but I'd like to know more about how determine the which one it will when both are active.

For every network interface there is an associated number called the metric, when routes get created for that interface they use that metric for the route.

When system needs to send a packet and needs to decide how to route the packet it consults the route table, working through the routes from most specific to lease specific. Whenever it finds two routes of identical specicifity it will use the route metric to decide which route to use.

See also:

How to choose which NIC to use for web surfing.

If you are asking if you can setup routes under windows to select the route by what type of protocol (http,ssh,etc) you are using, then I have to tell you that is not possible under Windows. You would need to setup up a router that supports policy-based routing. The LARTC howto has a very good discussion on how to do this under Linux.

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