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Default Gateway on multihomed server

By Bayk ·
Here?s a problematic situation I am encountering which must be quite common except that Microsoft?s implementation of TCP/IP turns it into a ridiculous situation.

I?m running a Windows NT server with three NICs.
I have configured two NICs in thefollowing way:

NIC 1 (on motherboard):
IP address 2.0.0.40
Default Gateway: 2.0.0.197

NIC2:
IP address 172.53.3.40
Default Gateway: 172.53.3.9

Each NIC is obviously on a different subnet. Each NIC is also on a separate LAN materialisedby two switches with no links between them. Each physical network has a separate router in the case of logical network 2.0.0.0 the router has the 2.0.0.9 address. In the case of the 172.53.3.0 logical network the router has a 172.53.3.9 address.
I believe this is a legitimate use of multiple NICs on a server. In effect the server collects data from the 2.0.0.0 network, it processes it and distributes the resulting data to network 172.53.3.0
The 2.0.0.9 route is used to allow remote administration of the 2.0.0.0 network constituents
The 172.53.3.9 router allows remote clients to recuperate data having been processed in the 172.53.3.0 network
Microsoft?s implementation of TCPIP allows only for one default gateway for the machine (see MS knowledge base Q157025) other gateways are considered to act as backup if the first gateway fails. This has proved to be an obstacle to the proper implementation of the server NIC configuration described above.

I am sure a lot of you out there have put in place a network distribution similar to the one described. Can anyone enlighten me on how it was done and what the resulting performance was like?

Happy New Year to you all

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Default Gateway on multihomed server

by shafique.awan In reply to Default Gateway on multih ...

I believe that I have had a similar problem.
It is true that the TCP/IP implementation means that there will only be one gateway. In order to solve this problem you can define a route table on your server. Look into the command "ROUTE" you can runit in the commmand window. So you will prehaps need to add a route command for your 2.0.0.0 network.

route 2.0.0.0 mask yoursubnetmask 2.0.0.40

This will redirect traffic to 2.0.0.0 to the network card of your choice and it will use the gateway for that card. I might not be very clear in my explanation but look into it.

You will note that this is not permanent and if/when you restart your servers you will have to do that again so put it in the startup or somewhere. I put it in the registery key "run".

You will keep your configuration of your cards as they are.

Regards

Feel free to contact me if you need.

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Default Gateway on multihomed server

by shafique.awan In reply to Default Gateway on multih ...

To Leopold
Thanks for pointing out the static entry parameter. I did not bother to read up onit and assumed that it could not be static. :)

Thanks again

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Default Gateway on multihomed server

by Bayk In reply to Default Gateway on multih ...

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Default Gateway on multihomed server

by Curacao_Dejavu In reply to Default Gateway on multih ...

Your thoughts and logic to add a default gateway are correct, however MS has a bug that allows only 1 default gateway even if you have more nics installed there can be only 1 default gateway.
answer #1 is correct in that.

In order to solve that you have to provide a static route for all other subnet masks.
to make the static route permanent add the "route -p ... " so the routing wont disappear after a reboot.

mskb Q141383

Leopold

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Default Gateway on multihomed server

by Curacao_Dejavu In reply to Default Gateway on multih ...

see laso ms kb Q159168

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Default Gateway on multihomed server

by Bayk In reply to Default Gateway on multih ...

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Default Gateway on multihomed server

by shmaltz In reply to Default Gateway on multih ...

Here are points to remember with Microsofts implementation of IP routing.
1. Only one (1) default gateway can be active on any system at any time.
2. If you do configure more than one default gateway windows assigns a metric to each gateway the one with the lowest gateway is always used first.
3. The default gatway is only used for networks that the routing table does not have a route to.
You can use the route command to adjust routes, or view them.
The reason for the above is very simple.

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Default Gateway on multihomed server

by shmaltz In reply to Default Gateway on multih ...

The reason for the above is very simple.
Since Windows IP routing will figure out the network (using the netmask) of the destination host. Windows will know if its on any of the directly attached interfaces (interface being any IP address configured for the local comptuer, regardless of how many adapters installed, since it's possible to have more than one IP address for one adapter, and more than one adapter for one IP address, its important to know this).
The easiest (but by no means the best) way to connect a windows machine to more than one network, would be by using only one adapter, one default gateway, and then have a router that performs the routing.
If you are required to set up a windows machine to be connected to more than one network, then you might not always be required to have more than one default gateway.
For example host1 is connect to net1 and net2 (using 2 NICs), net1 has a router (or gateway) that connects it to any other network or the internet. net2 has norouter or gateway connect to it (exept to connect to net1) there is no need to configure another gatway (besides the one on net1) for host1 since it can already access any network in the world that it has a phisical link to (with the gateway on net1).

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Default Gateway on multihomed server

by shmaltz In reply to Default Gateway on multih ...

The only time that you are required to configure an additional gateway for host1 whould be: If net2 is connected to net3 and the router on net1 doesn't know about net3 (or it doesn't have any means by which to connect), but the routere on net2 does know about net3. If this is the case than you most set up an additional gateway for host1 in order to be able to access net3. However this is not possible to do with setting up an additional DEFAULT gateway. Since as the name implies that if it doesn't know how to accesss the destination network it should use the defautl gateway. For which windows assumes that the gatway knows how to reach it. The abillity to set up more than one default gateway is only there if one of the gateways fail to connect to the destination host then windows will change the metric on the default gateways.
In order for host1 in the above example to be able to access net3 you will have to use the route add command.

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Default Gateway on multihomed server

by Bayk In reply to Default Gateway on multih ...

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