BTNet Router Dual WAN Subnets

Here is what is happening:

We have always had a Cisco Meraki and a DrayTek 3910

The Meraki runs an 81.x.x.x/31 subnet on WAN and a 31.x.x.x/27 LAN

The Meraki LAN is the DrayTek WAN which has the Meraki as the gateway and has lots of 192.x.x.x/24 subnets behind as well as services that use the 31.x.x.x/27 addresses to be publicly routable.

Now we are moving to wires only which means no Meraki

So the DrayTek is going to be the 81.x.x.x/31 but still needs to route through to the 192.x.x.x/24 subnets

Just to be clear, we are just removing the router, not changing ISP

So the Draytek must route to the 31.121.141.161/27 address when the WAN is on 81.134.210.124/31

Can we just add the 31.x.x.x/27 alias' to the WAN and will it still route?

If you just get the BT commissioning engineer to delete the 81.x.x.x subnet and route to the router as the first available address in the 31.x.x.x subnet you will not have to change anything.