OpenBSD default gateway outside subnet

There's a difference between forcing traffic through a gateway, and preventing devices from seeing each other on a subnet. I think what you're trying to do is the latter. Probably because you want to host servers for multiple external customers. You're looking in the wrong place if you're editing the routing table to achieve this goal.

My recommendation would be to handle this at the switch using ACLs. What specifically you can block/allow will depend on your switch's capabilities. Another alternative is to block ARP broadcasts, and then set static ARP entries for the default gateway (Technically someone could add more static entries to talk to other devices still).

Also, keep in mind that unless the gateway is set to block traffic from-to that segment it will happily route traffic right back into the same network.

Well to put it simply, the documentation is wrong. You cannot have a gateway that is not in "your" subnet. Since I think I understand what they meant, I'll try to correct it:

Dear Customer,

• you have been placed in a subnet 2a01:4f8:61:20e0::1/59.
• Please use a gateway 2a01:4f8:61:20e0::1, that is properly within your subnet.
• But please do not use IP addresses out of the range 2a01:4f8:61:20e1::1/64. These addresses are not for you.