I have connected a windows 2003 router with a linux fedora8 router.Each router serves a separate subnet, where I have 2 clients and an application server(dhcp,dns,ftp,web,mail) for each subnet. I have also added dhcp relay to each router and I try to test it.But first of all, note that I have 2 ethernet cards for each router.The one card is connected to the switch for the subnet, and I use the other for the connection between the routers.I use the right cables, and I also use IPs different from my subnets, in order to connect the routers
(192.168.17.0/30).BUT, the ethernet card of the windows router shows the green led, whereas the linux router card shows yellow.Do I miss something that I have to do with them? I also have some problems with pinging from the windows side to the linux side.I susspect that although my cards show "active" in linux router, I miss something..

It sounds like you are missing some type of a traffic policy to allow ICMP packets between the two different subnets. What do you get when you ping from Windows to Linux ? Host is not reachable or Packets Dropped, etc?

Host not reachable is the message, when I ping from my windows server e.g. to the linux server.But, linux server pings to windows server. ALso, Linux router(where dhcp relay relies), can ping to windows server to other subnet, but the opposite gives again host not reachable.

sounds like the linux box is blocking icmp pakets, (default in many linux distros) try tweaking your firewall settings to allow pings....

also with regard to newwork card LEDs every manufacturer does it different,

on mine for example, i get a green light for 1000mbs orange for 100mbs and no light for 10mbs with another orange light that signifies link and flashes on data