[FW1] static ip routing problem Help !!!!

[jos van Ditmarsch]

QUESTION: who can help me with the following problem : we have build a wan with 3com linkbuilder II isdn router andsuperstack II hubs. we have for example the following ip range in use: 192.168.100.1  default gateway and superstack ii isdn router 192.168.101.1 superstack II isdn router 192.168.102.2   "             " 192.168.103.3   "             " 192.168.104.1   "             " 192.168.100.11 unix machine 192.168.100.15 cobalt cube with isdn router to ISP                This machine is also running DNS service for                the workstation on range 192.168.100.1 the other range's where workstations are on have static ip numbers. On router 192.168.100.1 we use static route tables where all the subnets 101.1 102.2 are mentioned in the tables. From all the subnets we can go to the unix machine on 192.168.100.11. On range 192.168.100.??? through DNS al ip numbers for the workstations are correctly distributed. But now we have the following problem : When we add the following static route :  0.0.0.0 0.0.0.0 gateway 192.168.100.15 to router 192.168.100.1 , to force all the internet trafic to go to the cobalt cube (our ISP connection) , the default gateway becomes chanced in the route tables from 192.168.100.1 to 192.168.100.15 , which causes the problem that the workstations which have to access the unix machine can not find the unix box anymore. When we change the default gateway back to 192.168.100.1   everything works fine. The big problem is that it can happen every minute on the day. When we delete the route 0.0.0.0 0.0.0.0 then we dont have the problem. But then the internet connection wont work. I think it has something to do with RIP but I'm not sure. Can anyone give me a helpfull hint or tip how to solve this problem. Thanks in advance jos van Ditmarsch