If I set a /30 for subnet and enable dhcp, starting pool address will be the same as interface adreess. Since dhcp pool don't accept same starting and ending address, dhcp can't be configure correcly on a /30 subnet.
@Fedor - I haven't tested the /30 dhcp behavior but I think the issue that Mike brings up is that if a /30 is assigned to the interface handing out DHCP there is only one address available to assign to a connected device.
The way the GUI lists DHCP Start IP and DHCP End IP would imply that th eaddresses in that range including the start and end address can be assigned. For a /30 it would be logical to have the start and end address be displayed as the same IP as that is the only one actually available for assignment. This is how it would be displayed in most DHCP implementations.