DrayTek Vigor 3900/2960 Intersite/inter-subnet communications via direct connection on LAN side 

In this example we have two subnets

10.10.100.0/24 with a DrayTek Vigor 3900 at 10.10.100.1 and

10.10.101.0/24 with a DrayTek Vigor 2960 at 10.10.101.1 (Backup site.)

They are connected across a single cable through the datacentre providers' fibre backbone, configured to send all traffic to and from each port at each data centre.

We will configure the 3900 to have a dedicated port for a VLAN that has an IP address on the remote network of the 2960.

We add a static route to the 2960 through the IP address of the VLAN to connect with the 3900's LAN

Firstly, let's configure the 3900:

Go to LAN and General and create the new LAN:

We are creating the LAN with an IP of 10.10.101.254 as this is not in use in the remote site, note that we are not adding a gateway and disabling DHCP:

Once we have the LAN set-up, we must assign a dedicated LAN port on the 3900 to be the untagged member of that VLAN.

Go to VLANs, and configure as follows:

So we have made the LAN1 port an untagged member of our 10.10.101.0/24 VLAN and left the LAN2 port and SFP ports as untagged members of the oroginal 10.10.100.0/24 LAN.

This now gives us a port (LAN1 in this case, and there is only one other copper connection on a 3900 which means we cannot assign another VLAN untagged dedicated port unless we are using the SFP port) which is essentially a member of the remote subnet, allowing us to route traffic within the 3900 that will then be transported by the cable in LAN1 to the remote router subnet.

Now we need to tell the Vigor 3900 that the two subnets can intercommunicate, go to LAN > General set-up > InterLAN routing tab. 

Enable the two LANs to be able to communicate:

The 3900 subnet should now be able to ping the 10.10.101.1 router (if you have already connected the routers by cable) as the 3900's LAN1 port is effectively a member of the 10.10.101.0/24 subnet.

Now we need to tell the DrayTek 2960 how to find the 10.10.100.0/24 subnet. We do this by defining a static route:

Go to Routing > Static Route 

Add a static route that is to the 3900 subnet of 10.10.100.0/24 and define the gateway as the IP of the 3900 on the 2960's subnet, which in this case is 10.10.101.254. This will allow a return path for the 2960 to reach the 3900's subnet.

We can now route traffic successfully between the two subnets, and by using DNS and defining various active directory sites (assuming you are using Windows Server) you can have multi site single domain with multi-site backup facilities and without any confusion over which site receives DHCP from whom or which default gateway clients shouls be using.