Hi

Because I work from home I have divided my home network into two subnets, for simplicity's sake I'll call them Work and Home. I have turned off DHCP on the modem/router supplied by my ISP and added a raspberry pi as my DHCP server. I've attached a schematic of the network which shows how several devices are connected together. (The actual network is quite a bit more complex than this but I've reproduced the problems I'm having with the cut-down version shown in this diagram.)

All cables are cat6, both hubs are Dlink DGS1008D (hence gigabit), and every device is connected via a gigabit network connection except for the pi which is 100M.

I am having three issues which are probably related but wonder if anyone can help me with an explanation for what I'm seeing so that I can address them.

1. File transfer speeds are very different depending on the subnet.
a. Transfers between 192.168.0.4 (a Synology Diskstation) and 192.168.0.21 (my homebuilt desktop machine) run at 100Mb/s.
b. Transfers between 172.16.9.5 and 172.16.9.4 run at 10Mb/s.
c. Transfers between 172.16.9.5 and 192.168.0.4 run at 1Mb/s.

2. Although the pi is not on the same hub as either the XPS15 or the Diskstation, when copying files between them - (c) above - the pi receives approximately 10,000 IRQ32 interrupts per second. For comparison, when copying within either subnet - (a) or (b) above - the pi receives approximately 700 interrupts per second, which is the same number as it receives when the network is idle.

3. I don't know whether it is relevant or not but the only difference between the two subnets is that the DHCP service on the pi points machines on the Home subnet to do DNS lookups externally (using my ISP's name server or Google's as a fallback), while machines on the Work subnet use the pi so that they can resolve addresses in the VPN which my work machines are connected to (the pi is running openVPN to enable this). But even if the only activity in the network is copying my backup files from the work XPS15 to the home Diskstation, the 'named' (DNS) and 'openvpn' processes on the pi are CPU bound. When the backup finishes, they revert to 1% cpu at most. This means that when the backup *is* running any other network activity in the work subnet is prone to timeout, and connections to other sites in the VPN are dropped.

Issues 2 and 3 would cease to be problems if the backup transferred at the speed it should be capable of.

Anyone got any helpful ideas?

cheers
T