Subnets on single switch problem

Question

Hi i recently completed a CCENT and am now moving onto to CCNA. I came across a network recently shown in the attachment that had a problem and i have a few questions and am trying to debug it. Any suggestions would be appreicated.
Device a sends info to device b every second over tcp/ip (few packets of info). If the link to Switch 1 is not connected the 192.168.1 network works fine and device a and device b function as normal.
When the link from switch 3 is plugged into switch 1 after approx 2 hours device b stops working as it does not receive infomation from device a.
This is repeatable plugging into switch 1 always causes an issue on device b.
All the switches are unmanaged switches.
The server needs to communicate with the 10.0.1.x and to the 192.168.1.x network but the other deices on the 10 network and 192 network do not have to communicate with each other. The server runs monitoring software and needs to "see" all devices. The server has one NIC with an alternate IP set up and can ping the devices on the 192,.168.1.x network and the 10.0.1.x network.
I was wondering is there a potential issue with the two subnets on the switch or is there possibly an issue with broadcast traffic?
The devices are not PC but industrial communication devices so im am unsure how to debug the issue.Also cannot view any logs on the switches as they are unmanaged.
One though i had was to put a cisco 3550 switch i have for testing in there and connect device a,device b on the cisco switch, then create a SPAN port and connect wireshark/dumcap to the SPAN port to monitor the traffic being sent between the two.
If anyone has any suggestions please let me know thanks

Answer 1

One characteristic that is generally true of unmanaged switches is that they have no concept of VLANs and that all of their ports are all in the same VLAN (same broadcast domain). So although you may have some devices configured as part of 192.168.1 network and other devices configured as part of 10.0.1 network the reality is that all of the devices are part of one big flat network.
If you put a Layer 3 switch into the network then you create the opportunity to actually separate these networks. You could configure vlan 10 and assign some ports on the layer 3 switch as access ports in vlan 10. If you connect switches 2 and 3 to ports that are access ports in vlan 10 then all the ports in switches 2 and 3 will be in vlan 10. You could configure vlan 20 on the layer 3 switch and make vlan 20 have subnet 10.0.1.
You could connect switch 4 to an access port in whatever vlan you choose. You suggest that it might go into vlan 10 which is certainly possible. But you then suggest setting the IP of the server as part of 10.0. But in the preceding paragraph you suggested that 10.0 would be associated with vlan 20. You need to be clear and consistent about the relationship between vlan and subnet.
If you associate 10.0 subnet with vlan 20 then any device in vlan 20 should have an IP address in 10.0. And all devices in the vlan and subnet would easily communicate with each other.
I am somewhat unclear about the server. At some places you seem to indicate that it needs to communicate with devices in network 10.0 and in some places it seems that it should communicate with 192. If it really needs to communicate with both then you will need some layer 3 device that allows routing between the subnets.
HTH
Rick