From your network description, it appears that the server in question IS YOUR FIREWALL! The network makes perfect sense. Note, it appears that you may have multiple problems, not just a bad NIC issue.
The ISDN is your connection to the Internet,through the server (which is your hardware firewall), and the two other LAN connections to the switches are the Internet access through the server\Firewall in question. The cross connection between the two switches just reduces local traffic from having to route through the server for everything, reducing the load on the server, and the two NICS to the different switches improves the server performance by passing traffic directly from each switch (outbound only) through its own NIC access.
The network should work just fine as long as each switch uses it server NIC as the gateway for that switch, and the Server ONLY has a gatway address on the NIC to the ISDN.
This configuration should work fine even if one of the Switch/server link NICS goes bad, since the switches should be able to route around the NIC problem by going through the other switch via the uplink cable connecting the two switches.
First test: Verify that the server itself can browse the Internet via the ISDN link. Since you said half the network lost access, I do not expect this to fail. Then ping from the server to each of its NIC card addresses to verify the Server can see all three cards. Next ping from the server to the Switch port addresses to see if the issue is at the switch instead (or the cable connecting the two).
Next, refer to the following: Call the NIC0 the ISDN connection, NIC1 the NIC to switch 1, and NIC2 the NIC to and switch2. From a system on Switch1, ping in order to the IP address of NIC0, NIC1, NIC2. Do the same with a system connected to Switch2. If all work, it demonstrates the redundancy is working. If not, then your problem may be at the switch, not the server end.
A failed NIC toward the server (note: it could be either the Server or switch NIC) should not stop half of the network from internet access. A bad cross connect plus a bad Server link to either switch would block half the users. The routing tables in the switches should route Internet traffic to the server NIC first, then to the other switch second.
Now do the test again , but disconnect the link between the switches. If all still works, then all the NICs are accessable and working. If not, you will have found a possible bad connection between the switch which cannot ping and the server. Could be a bad port on the Switch, bad cable, or bad NIC at the server.
AT this point I would try a new NIC card or another switch port for the link which does not work. If the new NIC is now reachable, you still have the issue of why the uplink path does not work, and other than replacing the uplink cable between the switches (and checking that the port being used on each switch is correctly set to work as an uplink!!!) and testing again by disconnecting one switch at a time from their server link.
I suspect that your uplink is not working AND you have a NIC or server to switch connectivity issue. The Uplink only becomes obvious because of the failed NIC link and the failed uplink will block half of the users (those on the switch which has BOTH problems.).
Let us know what you find.
HTH
David
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